PsfexPsfDeterminer: ensure samples are completely configured

By instantiating and then not configuring samples (e.g., when
psfCandidate.getMaskedImage() threw an exception), the sample
remained active with nonsense values, picked up by valgrind,
and sometimes resulting in a PSF composed of NANs.

Made sample creation atomic; samples are effectively inserted
into the list when calling "finiSample".  Also enclosed sample
setup in a try/except/else block, to ensure we only finalise
on a sample that is suitably constructed.