Use specific error message for unique email constraint violation

[jyeshe]

Describe the bug

Currently all email conflicts and unique constraint violations generate an email error message:
"This email address already exists."

The message was generalized on workflows API work to report other field errors correctly, but email message needs to be a specific.

Version number

v2.10.9

I have reproduced this locally on main:

• Yes
• No

To Reproduce

1. Go to any screen with a field that is not an email and needs to be unique and uses the ChangesetJSON
2. Try to duplicate it and it shall show "This email address already exists."

Expected behavior

Use "This email address already exists." for email

Use "This value should be unique." for other fields (fixed on Workflows API)

[jyeshe]

Issue derived from #2792 (comment)