Better handle missing index file

[blankenberg]

Currently error doesn't provide good feedback:

Traceback (most recent call last):
File "/home/nickeener/anaconda3/envs/py2/bin/naive_variant_caller.py", line 94, in 
if name == "main": main()
File "/home/nickeener/anaconda3/envs/py2/bin/naive_variant_caller.py", line 89, in main
allow_out_of_bounds_positions=options.allow_out_of_bounds_positions, safe=options.safe )
File "/home/nickeener/anaconda3/envs/py2/lib/python2.7/site-packages/pyBamTools/genotyping/naive.py", line 93, in init
self._dtypes = guess_numpy_dtypes_from_idxstats( bam_readers, dtype, force_dtype=force_dtype )
File "/home/nickeener/anaconda3/envs/py2/lib/python2.7/site-packages/pyBamTools/util/init.py", line 41, in guess_numpy_dtypes_from_idxstats
refs = bam_index._references
AttributeError: 'Reader' object has no attribute '_references'

xref: #3