Remove heavy-handed failure mode in BamMultiReader::SetRegion

Remove heavy-handed failure mode in BamMultiReader::SetRegion

In practice a failure of BamReader::SetRegion means that we can't get
alignments from the specified region.  It is simpler to ignore failures
of SetRegion as they are gracefully handled by UpdateAlignments, which
simply doesn't add alignments from the readers which don't have
alignments in the target region.

This resolves a bug in which bamtools count (and any other utility using
BamMultiReader::SetRegion) would crash when provided a target region
with no alignments.