Allow `Context` to wrap `Fail` implementations about failure

[epage]

Naively, it'd be great to just make Context generic over a Fail. The problem is that Error isn't a Fail because of coherence problems.

Without resolving the coherence problem, either by waiting to break the API once specialization is available or by removing the From<Fail> for Error, this would probably require expanding the Either enum to a generic Fail. The problem with that is that Context would then have a useless generic parameter when used with Error.

[withoutboats]

This is an intentional design choice. The value of Context largely comes from the fact that you can wrap multiple kinds of errors together. If it had a parameter, this would not be true. Apologies for not explaining in more detail, but I explored down this path and found it added a lot of complexity to the APIs and to the recommended Error + ErrorKind pattern.

In contrast, if all you want to do is add some displayable info to a particular error type, the wrapper for that is not very difficult to write.