QmlV8DebuggerClient: Evaluate Expression

V8 engine may return an anonymous event for Evaluation of an expression
instead of a response, if the expression has syntax error. This fix
handles this particular case.

Change-Id: Ic89252b04eb4d6f7e6b0b2e03067f031f1f446ed
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>