Handle file/index not found errors in reader classes gracefully instead of segfaulting!

This has been a common complaint against gamgee of late, and
with very good reason!

Resolves #336
can you make a pull-request to htslib so we can get rid of this hack? (2 tickets)

it is likely that if we don't do it, someone will, and then we will be surprised with this in the future. For now this is okay.