errors: return more detailed errors when failing to parse or apply patch This patch adds subclasses of `PatchError` so we can distinguish between failure to parse a patch from failure to apply it. It updates the callers to raise either `InputError` or `StateError` depending on which type of error occurred. Differential Revision: https://phab.mercurial-scm.org/D11824

extensions: add a default "*" suboptions prefix This is similar to what we do in other section (e.g. `paths`) and allow to change the behavior for all extensions. Sub options on individual extensions overwrite the default settings. Differential Revision: https://phab.mercurial-scm.org/D11823

extension: add a `required` suboption to enforce the use of an extensions If `required` is set, failing to load an extensions will abort. See the test and documentation for details. Differential Revision: https://phab.mercurial-scm.org/D11822

extensions: highlight the name of the faulty extensions in the error message This make it easier to understand the message when the extensions name is common. Differential Revision: https://phab.mercurial-scm.org/D11821

extensions: refactor handling of loading error make it reusable We will need this in the next patch. Differential Revision: https://phab.mercurial-scm.org/D11820

extensions: ignore "sub-options" when looking for extensions config suboptions are separated by ":" (see the path one for example). So we dont want to confuse these config with actual extensions. We don't have extensions sub option yet, but I am about to introduce one for making sure an extensions can load. So lets level the floor first. Differential Revision: https://phab.mercurial-scm.org/D11819