ui: change default path fallback mechanism (issue4796) The previous paths API code always fell back to the default path. This was wrong because if a requested path doesn't exist, that should error. Only if no path was requested should we fall back to the default. As part of implementing the test case for issue 4796, it was discovered that the "repository does not exist" error message raised by localrepository.__init__ wasn't being seen because the paths API validates paths before localrepository.__init__ was being called. The exception and error message from localrepository.__init__ has been introduced to getpath(). This necessitated rewriting expandpath() both to catch the exception and to have proper default fallback. This code is more complicated than I'd like. But making all tests pass was a big chore. As more code moves to getpath(), there will likely be opportunities to improve things a bit.