Fix assertion: ASSERT: "hasException" in file jsruntime/qv4engine.cpp, line 933

This was seemingly tripped by trying to catch the exception twice: once with
catchException and once with catchExceptionAsQmlError.

Change-Id: I7176d56fe6e6f748e80d0894e314ed2b8f6e751d
Done-by: Mikko Harju <mikko.harju@jollamobile.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>