Let qbs generate check if the generator exists before "building"

When executing qbs generate -g <invalidname>, qbs does some heavy processing before checking whether the generator is really valid. In case of Qt Creator, it takes qbs a minute to fail. If possible, it would make sense to do the check beforehand.

c:\Users\aportale\dev\qt-creator>c:\Qt\qtcreator-4.5\bin\qbs generate -g schnüselgrumpf

Restoring build graph from disk
Resolving project for configuration default
Building for configuration default <== takes a while!
Build done for configuration default.
ERROR: No generator named 'schn³selgrumpf'. Available generators:
clangdb
visualstudio2010
visualstudio2012
visualstudio2013
visualstudio2015
visualstudio2017