[Fix #800] Do not report [Corrected] if correction wasn't done

A few cops that support auto-correction avoid it in certain cases.
They should only print [Corrected] if a change was made. Solution is
to reintroduce the exception CorrectionNotPossible that was removed
a while back.
Merge pull request #962 from jonas054/fix_800_corrected

[Fix #800] Do not report [Corrected] if correction wasn't done