bundle2: introduce a PushkeyFail error to abort unbundle on pushkey error
The pushkey code is generic and the server side has little context on what the
client is trying to achieve. Generating interesting error messages server side
would be challenging. Instead we introduce a dedicated exception that carries more
data. In particular, it carries the id of the part which failed that will allow
clients to display custom error messages depending on the part intent.
The processing and transfer-over-the-wire of this exception is to be implemented
in coming changesets.