CtfVisualizer: Do not crash on unexpected types again

And print a nicer error message.

Amends c05f9cacc6

Task-number: QTCREATORBUG-29659
Change-Id: I1db6bea0bedf1fae034fecbbbeae56bb2fee49ed
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/ctfvisualizer/ctftracemanager.h

@@ -46,6 +46,8 @@ public:
     void updateStatistics();
     void clearAll();
 
+    QString errorString() const;
+
 signals:
     void detailsRequested(const QString &title);
 
@@ -66,6 +68,7 @@ protected:
     double m_traceEnd = std::numeric_limits<double>::min();
     double m_timeOffset = -1.0;
 
+    QString m_errorString;
 };
 
 } // namespace Internal