Use a warning if a remote bam file has no index.

Hi,

I was trying to curate some ENCODE3 data on a data warehouse that only had the .bam files and not the .bai files. Since I just needed to read the header and a few reads, I could avoid the samtools functionality that needs the index.

So I changed the exception to a warning. Do you think it would be reasonable to incorporate this patch?

Diane