bundle2: rename b2x:error:unknownpart to b2x:error:unsupportedcontent This is a backward compatibility breakage per se. But bundle2 was explicitly flagged as experimental, and this is one an error path anyway. So the worse possible outcome from this change is to still have a crash but with a different message.

bundle2: move exception classes into the error module Exceptions should have known their place.

bundle2: rename UnknownPartError to BundleValueError We are going to raise exceptions for a wider range of cases: unsupported mandatory stream and part parameters. We rename the exception with a wider name.

context: explicitly return a tuple In the refactoring of removing localrepo.status, 2edb8648c500, we accidentally changed the return type from a tuple to a list. Philosophically, this is incorrect so we explicitly return a tuple again.