errors: create superclass for Abort exception I'd like to let extensions subclass `StorageError` to define a custom exit code. However, `StorageError` does not extend `Abort` (which is where the exit code currently lives), and it seems that it's not supposed to either (`StorageError` seems to be for lower-level errors and `Abort` is for command-level errors). This patch therefore extracts all the code from `Abort` into a new `Error` class, which I'll soon make `StorageError` also extend. Differential Revision: https://phab.mercurial-scm.org/D10738