use 'thiserror'

[logannc]

The wisdom for a while has been "anyhow" for binaries, "thiserror" for libraries. The macros from thiserror are simply and non-magic enough that I feel comfortable using them normally.

Originally posted by @seanpianka in #28 (comment)

https://github.com/dtolnay/thiserror

we have a pretty small error api surface but might as well