External errors which aren't used as an error in the original crate don't work correctly.

[mhammond]

eg, consider crate-1 which defines an error enum, but where crate-1 doesn't actually use the error locally. When crate-2 tries to use that error enum it fails because crate-1 didn't generate everything needed for errors, because due to internal implementation details, it simply doesn't know it is an error.

This can be demonstrated by the patch in #2390 - it added a new exported function _just_to_get_error_support() purely to work around this. While this is also a semi-reasonable work-around, it's far from ideal.