handle missing blast results file

[jchorl]

When blastn produces empty results, it deletes the file: https://github.com/phac-nml/mob-suite/blob/6fa359789a04877cc6d7863f380f31816480b43a/mob_suite/utils.py#L344

However, the calling function, blast_mge, checks for an empty results file. When that file doesn't exist, it throws a file-not-found exception. This feels like a bug.

Instead, handle gracefully. An alternative approach would be to get the response from blastn (True or False) and use that. I don't know that either is necessarily better.