dinitctl: slight improvement to err msg when no instance running

It seems like it's common enough for users to be confused when run a
dinitctl command with no user instance, so slightly improve the message
(to suggest the reason for socket connection failure might be the
absence of a user instance). It might not be enough to resolve all
confusion but it is at least an improvement.

src/dinitctl.cc

@@ -531,10 +531,13 @@ int dinitctl_main(int argc, char **argv)
         }
     }
 
+    bool user_specified_cs_path = false;
+
     if (!use_passed_cfd) {
         // Locate control socket
         if (!control_socket_str.empty()) {
             control_socket_path = control_socket_str.c_str();
+            user_specified_cs_path = true;
         }
         else {
             control_socket_path = get_default_socket_path(control_socket_str, user_dinit);
@@ -626,6 +629,14 @@ int dinitctl_main(int argc, char **argv)
     catch (dinit_protocol_error &e) {
         cerr << "dinitctl: protocol error" << endl;
     }
+    catch (control_sock_conn_err &ce) {
+        cerr << "dinitctl: " << ce.get_action() << ": " << ce.get_arg() << ": " << strerror(ce.get_err()) << "\n";
+        if (user_dinit && ce.get_err() == ENOENT && !user_specified_cs_path) {
+            // It is common enough that users don't realise they need to have a user instance
+            // running in order to control it, so elaborate a little:
+            cerr << "dinitctl: perhaps no user instance is running?\n";
+        }
+    }
     catch (general_error &ge) {
         std::cerr << "dinitctl";
         if (ge.get_action() != nullptr) {

src/includes/dinit-client.h

@@ -58,6 +58,15 @@ class general_error
     std::string &get_arg() { return arg; }
 };
 
+// Specialise general_error for the case of connecting to the control socket
+class control_sock_conn_err : public general_error
+{
+public:
+    control_sock_conn_err(int err, std::string sockpath) : general_error(err, "connecting to socket", std::move(sockpath))
+    {
+    }
+};
+
 // static_membuf: a buffer of a fixed size (N) with one additional value (of type T). Don't use this
 // directly, construct via membuf.
 template <int N> class static_membuf
@@ -333,7 +342,7 @@ inline int connect_to_daemon(const char *control_socket_path)
     free(name);
 
     if (connr == -1) {
-        throw general_error(errno, "connecting to socket", control_socket_path);
+        throw control_sock_conn_err(errno, control_socket_path);
     }
 
     return socknum;