There are still a couple of pain points here (EntryReader, for example),
but the general layout works pretty well. With error chain and one or
two handwritten impls I can pass a bare &Path to chain_err and it will
construct the correct error variant and add it to the chain. This is a
significant reduction in boilerplate.

I still need to do some manual testing to generate some error
conditions, but I think this setup will be more robust in the long term.