fix: add a config to abort when a fixer tool fails
This allows users to stop and address tool failures before proceeding, instead
of the default behavior of continuing to apply any tools that didn't fail. For
example, a code formatting tool could fail if you have syntax errors, and you
might want your repo to stay in its current state while you fix the syntax
error before re-running 'hg fix'. It's conceivable that this would even be
necessary for the correctness of some fixer tools across a chain of revisions.
Differential Revision: https://phab.mercurial-scm.org/D5200