Debugger: Clear message on attach to core

If a usable engine was found, there is no reason to display errors
set by another engines

Task-number: QTCREATORBUG-7150

Change-Id: I50cec27a127a0af34b5287c389df6ce6a797bb3e
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/debugger/debuggerrunner.cpp

@@ -747,6 +747,7 @@ DEBUGGER_EXPORT ConfigurationCheck checkDebugConfiguration(const DebuggerStartPa
     QList<DebuggerEngineType> unavailableTypes;
     foreach (DebuggerEngineType et, requiredTypes) {
         if (canUseEngine(et, sp, cmdLineEnabledEngines, &result)) {
+            result.errorDetails.clear();
             usableType = et;
             break;
         } else {