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.