QtVersion: Set qmakeIsExecutable to false if qmake crashes.

This prevents literally dozens of crash dialog boxes on Windows.
Check for a crash in the helper library and in updateVersionInfo
consistently whatever is called first.
Initialize qmakeIsExecutable in 2nd constructor as well.

Change-Id: Ifafb7b3edfae8826ac062d634a39a2a804446942
Reviewed-on: http://codereview.qt.nokia.com/1661


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>