Web client throws unclear error when the logged in user missed on SP

[IskandarovRost]

Environment

• Platform version: 6.8.7
• Client type: Web

Description of the bug or enhancement

• Minimal reproducible example

1. Adjust CUBA SSO as written in the documentation https://doc.cuba-platform.com/manual-6.8/sso.html
2. Start IDP
3. Start SP
4. On IDP and in LDAP: create some user. Do not create the user on SP.
5. In SP: login with the user credentials

• Expected behavior
  Some clear message.

• Actual behavior
  

2018-05-07 17:25:51.238 INFO  [http-nio-8082-exec-2] com.haulmont.cuba.security.auth.AuthenticationServiceBean - Login failed: com.haulmont.cuba.security.global.LoginException: Unknown login name or bad password 'user3'
2018-05-07 17:25:51.256 ERROR [http-nio-8082-exec-2] com.haulmont.cuba.web.AppUI - Unable to init ui
com.haulmont.cuba.security.global.LoginException: Unknown login name or bad password 'user3'
	at com.haulmont.cuba.security.auth.providers.TrustedClientAuthenticationProvider.authenticate(TrustedClientAuthenticationProvider.java:69) ~[na:na]
	at com.haulmont.cuba.security.auth.AuthenticationManagerBean.authenticateInternal(AuthenticationManagerBean.java:253) ~[na:na]
	at com.haulmont.cuba.security.auth.AuthenticationManagerBean.login(AuthenticationManagerBean.java:118) ~[na:na]
	at com.haulmont.cuba.security.auth.AuthenticationServiceBean.login(AuthenticationServiceBean.java:86) ~[na:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_151]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_151]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_151]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_151]
	at com.haulmont.cuba.core.sys.remoting.LocalServiceInvokerImpl.invoke(LocalServiceInvokerImpl.java:94) ~[na:na]
	at com.haulmont.cuba.web.sys.remoting.LocalServiceProxy$LocalServiceInvocationHandler.invoke(LocalServiceProxy.java:154) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.sun.proxy.$Proxy24.login(Unknown Source) ~[na:na]
	at com.haulmont.cuba.web.security.providers.ExternalUserLoginProvider.loginMiddleware(ExternalUserLoginProvider.java:91) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.security.providers.ExternalUserLoginProvider.login(ExternalUserLoginProvider.java:87) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.security.ConnectionImpl.loginInternal(ConnectionImpl.java:209) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.security.ConnectionImpl.login(ConnectionImpl.java:89) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.security.idp.IdpLoginLifecycleManager.onAppStarted(IdpLoginLifecycleManager.java:94) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_151]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_151]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_151]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_151]
	at org.springframework.context.event.ApplicationListenerMethodAdapter.doInvoke(ApplicationListenerMethodAdapter.java:256) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.context.event.ApplicationListenerMethodAdapter.processEvent(ApplicationListenerMethodAdapter.java:177) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.context.event.ApplicationListenerMethodAdapter.onApplicationEvent(ApplicationListenerMethodAdapter.java:140) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:393) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:347) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at com.haulmont.cuba.core.sys.EventsImpl.publish(EventsImpl.java:33) ~[cuba-global-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.sys.WebEvents.publish(WebEvents.java:36) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.DefaultApp.publishAppStartedEvent(DefaultApp.java:225) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.DefaultApp.loginOnStart(DefaultApp.java:212) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.AppUI.setupUI(AppUI.java:297) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.haulmont.cuba.web.AppUI.init(AppUI.java:221) ~[cuba-web-6.9-SNAPSHOT.jar:6.9-SNAPSHOT]
	at com.vaadin.ui.UI.doInit(UI.java:693) [vaadin-server-7.7.13.cuba.9.jar:7.7.13.cuba.9]
	at com.vaadin.server.communication.UIInitHandler.getBrowserDetailsUI(UIInitHandler.java:222) [vaadin-server-7.7.13.cuba.9.jar:7.7.13.cuba.9]
	at com.vaadin.server.communication.UIInitHandler.synchronizedHandleRequest(UIInitHandler.java:74) [vaadin-server-7.7.13.cuba.9.jar:7.7.13.cuba.9]