Change error type for redirect parsing errors (#1649)

* Change error type for redirect parsing errors

This changes the error type returned when the proxy fails to parse the
redirect target to be a 400 error instead of a 500 error.

As far as I can tell, the only way that this can fail is a failure to
parse the properties of the request to identity the redirect target.
This indicates that the user has sent a malformed request, and so should
result in a 400 rather than a 500.

I've added a test to exercise this, based on a real work example.

* Update changelog

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>