crate: add features to switch Error backend

[Fishrock123]

Allows end users to choose between anyhow and eyre.

If using eyre, it is up to the application author to install an eyre reporter themselves.

This is an updated rebase of #215, with some notable changes:

• No longer does an eyre reporter install.
• Is all behind feature flags.

A concern: adding error_anyhow could break people using default-features = false.

[Fishrock123]

Upon slightly further investigation - I think Tide may need to hook into this more manually in it's logger (or in any custom logger). More investigation is needed.