errors: move Abort earlier, so more exceptions can subclass it
I'd like to make at least `InterventionRequired` subclass `Abort` and
Python requires the superclass to be defined before the subtype.
Differential Revision: https://phab.mercurial-scm.org/D10736
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate