Cleaned up error logging for session creation failures

The old message logged the function name and thread ID which are not useful
to the end user. The thread ID should only be printed for debugging purposes,
not general error messages.

server/core/session.c

@@ -154,12 +154,8 @@ session_alloc(SERVICE *service, DCB *client_dcb)
         if (session->router_session == NULL)
         {
             session->state = SESSION_STATE_TO_BE_FREED;
-
-            MXS_ERROR("%lu [%s] Error : Failed to create %s session because router"
-                      "could not establish a new router session, see earlier error.",
-                      pthread_self(),
-                      __func__,
-                      service->name);
+            MXS_ERROR("Failed to create new router session for service '%s'. "
+                      "See previous errors for more details.", service->name);
         }
         /*
          * Pending filter chain being setup set the head of the chain to