(potentially for 5.12.1?) Provide clear error messages for remotely-authenticated login failures

This is a child issue for #9088; a quick bandaid fix for a confusing user experience with shib logins that fail because either the information from their institutional auth provider no longer matches what's stored in our records, or there is already a user account with the same email address but authenticated via a different method (github, orcid, etc.). This is described as option 2 in the parent issue. Option 3 would be a proper, longer term solution (basically, there are situations where it should be possible and safe to fix this problem automatically and transparently for the user).

Currently Dataverse shows them a new account form, and then of course it fails to save this new account, because of the unique constraint on the email (Shib users cannot choose a different email!). The error message shown to the user simply says "can't save account".

This experimental branch (built off the v5.12 tag) has a proof-of-concept fix: https://github.com/IQSS/dataverse/commit/de1730db4a05521d0c3bd82a6528b3eb6d5aa098. I didn't bother putting the text of these error messages into the bundle, because this is experimental and, presumably, temporary. I have tested this on demo. If we agree to include this into v5.12.1, I'll create a different branch, and a PR into the develop branch.

Start by reading parent issue #9088 and the proof-of-concept commit de1730db4a05521d0c3bd82a6528b3eb6d5aa098, which was built from the v5.12 tag. Review the remotely authenticated login failure cases described here and verify that users receive distinct, clear errors instead of "can't save account" when identity data conflicts or the email belongs to another authentication method.