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
Merge pull request #337 from broadinstitute/dr_fix_additional_reader_segfaults_on_nonexistent_files

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