shelve: rewrite check for unknown shelf to delete The code would try to delete the shelf's .patch file and if that raised an exception, it would convert it to an `error.Abort`. This patch rewrites it so the check is done upfront. I find it easier to read that way. It's now clear enough that I removed the comment explaining it as well. As Joerg pointed out during review, another differences is that the old code would move a `.hg` file without its `.patch` friend to backup before it realized that the `.patch` file was missing. The new code will error out earlier and not move the `.hg` file, which seems like an improvement. That should only matter on corrupt `.hg/shelved/` directories, however. Differential Revision: https://phab.mercurial-scm.org/D9697