Ugly Error When Username Isn't Unique

Steps to Repro:

1. Log into folio-snapshot-stable
2. Go to users
3. Click on any user and copy the username
4. Edit another user and enter the username you copied from the previous user record
5. Click to save

Expected: Error should be UI error, not a server one. Let's use "Username must be unique"

Actual: Ugly server error.