Mutagen source has large number of unhandled errors

[mikeschinkel]

I discovered these unhandled errors while I was making changes for my own needs on my fork for #505.

My IDE kept complaining about unhandled errors, so I decided to fix them and move to a separate PR as #508.

Serendipitously DoltHub published a blog post entitled "What's the best Static Analysis tool for Golang?" that starts with an example of an unhandled error as justification for why handling all errors is so important. I say "serendipitously" because it was published after I wrote the code for this PR.