[ENH] Fit-Check added to NiftiLabelsMasker

Contrary to NiftiMasker, NiftiLabelsMasker.transform() does not currently check whether fit() of the current instance has already been called. Added exception for this case.
Could you:
  - use assert_raises_regexp to check that the error message is the one you are expecting
  - add the same test for inverse_transform for completeness