The QmlDebugConnection can be quite spammy as it repeatedly tries to
connect to the application. Show error messages from this process in the
debugger log and only output a line in the status bar if the connection
is considered to have permanently failed.

Change-Id: I4a5715d9acdac47f690cb8aa67adb5cccff76e11
Task-number: QTCREATORBUG-17492
Reviewed-by: hjk <hjk@qt.io>