changectx: move `IndexError` handling in the top level try except This one can be handled by the top level catching.

changectx: wrap the `changeid` processing in a try/except We are going to introduce more precise exception classes for filtered nodes. So we will have to upgrade them to the `RepoLookupError` level here. We wrap the whole thing into a try/except to ease this future catching. Some of the current exception catching will be moved in this one. But the current changeset focuses on code movement only.

bundle2: detect and disallow a negative chunk size We have no usage planned for 2/3 of them and the support for the planned usecase is not here yet. So we raise a BundleValueError when encountered

bundle2: add an UnsupportedPartError We need the BundleValueError for format errors not related to part support. So we add a specific class for part-support errors.

bundle2: change header size and make them signed (new format) We are changing all integers that denote the size of a chunk to read to int32. There are two main motivations for that. First, we change everything to the same width (32 bits) to make it possible for a reasonably agnostic actor to forward a bundle2 without any extra processing. With this change, this could be achieved by just reading int32s and forwarding chunks of the size read. A bit a smartness would be logic to detect the end of stream but nothing too complicated. Second, we need some capacity to transmit special information during the bundle processing. For example we would like to be able to raise an exception while a part is being read if this exception happend while this part was generated. Having signed integer let us use negative numbers to trigger special events during the parsing of the bundle. The format is renamed for B2X to B2Y because this breaks binary compatibility. The B2X format support is dropped. It was experimental to allow this kind of things. All elements not directly related to the binary format remain flagged "b2x" because they are still compatible.

bundle2: extract processing of part into its own function This is code movement only. This will be useful to have it separated for reuse purposes. We plan to introduce a new feature to the bundle format that allow inserting a part in the middle of another part payload. This will be useful to transmit a exception raised during a part generation.

bundle2: add a test for exceptions raised during the generation process We would like exceptions raised during the generation process to be gracefully handled on the receiver side. We add a test for it. It shows that we are not doing it yet.