Use anyhow for error handling

[Turbo87]

see https://github.com/dtolnay/anyhow

This seems easier than defining our own Error enum, and it allows us to easily add additional context to the errors when they bubble up the stack.

Since this is "application code" I assume that https://github.com/dtolnay/anyhow#comparison-to-thiserror applies and that anyhow is the better solution for us, compared to using something like thiserror.

r? @jtgeibel

/cc @dtolnay

[jtgeibel]

This looks great to me! When I split these errors out from AppError I hoped to eventually pick a good library for this. This seems like a fine solution.

I'm going to wait to merge until after the next deploy, because we already have a huge backlog pending.

[jtgeibel]

@bors r+

[bors]

📌 Commit 52fdcc7 has been approved by jtgeibel

[bors]

⌛ Testing commit 52fdcc7 with merge be1c127...

[bors]

☀️ Test successful - checks-travis
Approved by: jtgeibel
Pushing be1c127 to master...