filemerge: store error messages in module variables
Copytracing may be disabled because it's too slow (see
experimental.disablecopytrace config option). In that case user may get errors
like 'local changed FILE which other deleted'. It would be nice to give user a
hint to rerun command with `--config experimental.disablecopytrace=False`. To
make it possible let's extract error message to variables so that extension may
overwrite them.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).