Don't expand to invalid Scops with -polly-detect-keep-going

Enabling -keep-going in ScopDetection causes expansion to an invalid
Scop candidate.

Region A     <- Valid candidate
   |
Region B     <- Invalid candidate

If -keep-going is enabled, ScopDetection would expand A to A+B because
the RejectLog is never checked for errors during expansion.

With this patch only A becomes a valid Scop.