Custom errors (#977)

* Implement support for custom errors

* Remove unneded for<'a> from E bound

* Fix doctest

* Handle the case where there are not exactly two arguments

* Support for other Result paths

* Rewrite with a more explicit rewriting logic

* Back to rewriting the error argument

* Add UI error for non-result

* Apply suggestions from code review

Co-authored-by: Niklas Adolfsson <[email protected]>

* Fix a typo

* Fix errors in the rest of the targets

Co-authored-by: Niklas Adolfsson <[email protected]>