* Fix old migration script not being able to run if it fails midway

Improve the robustness of a migration script likely to fail because of database
corruption so it can run again once database corruptions are fixed.

* Display a specific error message in case of index corruption

Co-authored-by: Eugen Rochko <eugen@zeonfederated.com>
Co-authored-by: Claire <claire.github-309c@sitedethib.com>

This should not be an issue in practice because of the Rails-level uniqueness
check, but local accounts having a NULL domain means the uniqueness constraint
did not apply to them (since no two NULL values are considered equal).