Fix WebAPIError.with_message, and add WebAPIError.with_override.
WebAPIError.with_message modified the main instance of an error, instead

of creating a copy. This led to pollution of the error instances. In

practice, Djblets and Review Board aren't bit by it, by this may change

over time. Now it creates a copy.
Since it's also useful to override headers, a new with_override function

has been added that takes message and headers. with_message now just

wraps this.

New unit tests pass.