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
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present