[General] Error handling, response strings, formatting

[asw101]

This is a general follow-up to #111. See the last three commits in particular.

• It applies similar error handling elsewhere in the code base (avoiding log.Fatal, returning errors). [commit]
• I've done a first pass over response strings for things like Github vs GitHub, etc. [commit]
• Ran go fmt ./... on everything (I was noting formatting changes from previous commits). [commit]

@imiller31 I've submitted this as a draft PR. Does it look reasonable, or is there anything it would be helpful for me to pay further attention to?

[asw101]

@imiller31 thank you for the feedback. Changes pushed.

[asw101]

@imiller31 I resolved the merge conflicts but deleted the errors-2 branch accidentally. I can PR again if helpful? https://github.com/asw101/draft/tree/errors-2