Show more useful error dialog in case of plugin errors.

Task-number: QTCREATORBUG-3940

Change-Id: I9e12ad68bba9c2f0d8f5243ab7dc763765fba756
Reviewed-on: http://codereview.qt.nokia.com/3811
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@nokia.com>

src/libs/extensionsystem/pluginmanager.h

@@ -106,6 +106,7 @@ public:
     QHash<QString, PluginCollection *> pluginCollections() const;
     void setFileExtension(const QString &extension);
     QString fileExtension() const;
+    bool hasError() const;
 
     // Settings
     void setSettings(QSettings *settings);