bundle2: convey PushkeyFailed error over the wire We add a way to convey the precise exception. This will allow better error message on the server.

bundle2: also capture reply capability on failure When unbundling over the wire is aborted, we have a mechanism to convey the error inside a bundle part. As we add support for more errors, we need to know if the client will support them. For this purpose, we duck punch the reply capabilities of the client on the raised extensions. This is similar to what is done to salvage the server output on error.