bundle2: introduce a parttype attribute to BundleValueError We will use the Exception for more that just unknown part type.

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.

wireproto: expose the list of getbundle arguments to extensions For now, getbundle accepts a fixed number of arguments: ``heads``, ``common`` and ``bundlecaps``. We make this list exposed at the module level to let extensions add content there. This is important for extensions that wish to use bundle2 for other contents than changegroup.

run-tests: write .err files earlier Earlier refactoring of run-tests.py accidentally broke --interactive and external diff generation by not having .err files written before they are consulted. This patch fixes that.

run-tests: exit with non-0 exit code when tests fail or warn As part of the run-tests.py refactor, run-tests.py accidentally started exiting with 0 for most test runs. This patch restores the expected behavior.

bundle2: expose mandatory params in a mandatorykeys attribute We expose all keys that MUST be processed in ``part.mandatorykeys``. This makes it much easier to access the information. Enforcement of the mandatory parameters is coming in later changesets.

bundle2: use the new ``part.params`` dictionary We use the new ``part.params`` dictionary to access the value of parameters instead of creating one from the part's attributes.