CMake: Throw errors for SIP problems in subsequent builds also

Old behavior was to only spit out errors on the first "make" run.
To reproduce the problem:

Edit a random sip file and introduce some parse error (e.g. append some garbage at the end of the file)
make (errors...)
make (again, no errors, start qgis: python errors indicating e.g. initgui not found)
To fix this you had to `touch python/gui/gui.sip`, make again and then solve the problem.

With this patch introduced, `make` will fail repeatedly untill you properly fixed the sip problem.