Warn the user if they attempt to open a remote bam file that has no index.
Warn the user if they attempt to open a remote bam file that has no index.

There are some use-cases where you might actually want to read a
remote bam file that has no index. The previous behavior stopped
with an exception, this patch instead reports the error as a
warning and continues.

daler/pysam,mattions/pysam,dannon/pysam,carolinux/pysam,kyleabeauchamp/pysam,TyberiusPrime/pysam,brendanofallon/pysam,mattions/pysam,carolinux/pysam,bioinformed/pysam,carolinux/pysam,carolinux/pysam,pysam-developers/pysam,daler/pysam,pysam-developers/pysam,mattions/pysam,nlhepler/pysam,mattions/pysam,TyberiusPrime/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,nlhepler/pysam,dannon/pysam,nlhepler/pysam,mattions/pysam,TyberiusPrime/pysam,TyberiusPrime/pysam,brendanofallon/pysam,kyleabeauchamp/pysam,pysam-developers/pysam,TyberiusPrime/pysam,daler/pysam,brendanofallon/pysam,bioinformed/pysam,bioinformed/pysam,dannon/pysam,dannon/pysam,brendanofallon/pysam,bioinformed/pysam,bioinformed/pysam,daler/pysam,pysam-developers/pysam,dannon/pysam,daler/pysam,carolinux/pysam,nlhepler/pysam,brendanofallon/pysam,nlhepler/pysam,kyleabeauchamp/pysam