Error constructor methods should return Self instead of Result<(), Self>

[fitzgen]

There are places where we want to return an error in a method that returns Result<Something, Error> and are forced to do nasty things like Error::crate_config("...").map(|_| unreachable!()) to make it type check. At this point, it would be easier to just do Err(Error::crate_config("..")) everywhere after changing the Error constructors to return Self.

[data-pup]

I'd love to help out with this one! I remember doing this for #244, and pulling it out so the PR would be a little cleaner. I don't think it would be hard to do again 😸

[fitzgen]

Great! ^_^

[ashleygwilliams]

@fitzgen now that we've shifted to using failure nearly entirely- is this out of date?

[fitzgen]

Yeah, I think this can be closed now!