bundle2: gracefully handle PushRaced error during unbundle Same drill again. We catch the PushRaced error, check if it cames from a bundle2 processing, if so we turn it into a bundle2 with a part transporting error information to be reraised client side.

bundle2: add an error message to push race error Errors with no explanations makes my uncle Bob sad.

bundle2: fix raising errors during heads checking If the heads on the server differ from the ones reported seen by the client at bundle time, we raise a PushRaced exception. However, the part raising the exception was broken. To fix it, we move the PushRaced class in the error module so it can be accessible everywhere without an import cycle. A test is also added to prevent regression.

bundle2: gracefully handle UnknownPartError during unbundle Same as for Abort error, we catch the error, encode it into a bundle2 reply (expected by the client) and stream this reply. The client processing of the error will raise the exception again.