Specify handler name in handler config error msg

[vyzigold]

Before:

[ERROR] transport handlers failed to load [transport: dummy-logs0, error: missing or incorrect configuration fields --  messageField , timestampField , hostnameField , severityField --]

Now:

[ERROR] transport handlers failed to load [transport: dummy-logs0, error: failed configuring handler plugin 'logs': missing or incorrect configuration fields --  messageField , timestampField , hostnameField , severityField --]

Solves issue #49

[vyzigold]

@leifmadsen should I cherry-pick this to stable branches?

[leifmadsen]

@leifmadsen should I cherry-pick this to stable branches?

No need. It'll get merged to stable-1.5 at the next z-stream. Thanks!