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.

bundle2: introduce a ``params`` dictionary on unbundled parts This exposes all parameters the part received into a ``part.params`` dictionary. This should be much easier to use. This dictionary itself does not expose the mandatory or advisory aspect of parameters, but no current users of bundle2 actually enforce any of this logic. Coming changesets will improve this aspect.

bundle2: make sure unbundled part param are read-only My old uncle Robert once trusted an API user. We never saw him again.

bundle2: introduce an ``_initparams`` method The handling of parameters will become much more sophisticated in the coming changesets. So we extract the logic in a function to not pollute the generic logic.