Ah oops, I just realized I accidentally rebased this patch on master.  I intended this to be on release-2.0.x.  Should I upload a new patch, or will you backport on commit?

So my use case is to provide slightly more detailed error messages to the user than just "publish error".  It might be "authentication expired" or "you don't have permission to write to the associated bug" or even "Bugzilla appears to be down".  In general, since the publish hook is meant for extensions, I think it makes sense to allow the extensions to define their own errors.  I could provide a generic PublishError subclass of WebAPIError to use the same error code (I don't particularly care about the error code value) which accepts and arbitrary message (and maybe http code).

Errors can have custom messages and payloads, so a hook could conceivably provide additional JSON to provide as part of that. I think that's probably the way to go, since we're talking custom code on both the provider and consumer ends at this point.
I would agree that the root error for this should be a PUBLISH_ERROR, but optionally with the custom payloads.
We always pre-define errors in errors.py, so we shouldn't manually create a WebAPIError anywhere outside that file.

Aha, I didn't notice the WebAPIError.with_message() method.  I'll pass custom PublishError messages with that.