Thanks, Eric. Reposting my comment so images show.
There seems to be a regression of a worser kind in Service Desk 3.10.2 (and possibly other versions).
If your validator just throws a plain old exception with no field specified, you get a super generic error in the service desk portal that doesn't contain your custom message:
throw new InvalidInputException("Summary is incorrect")

If you do go to the trouble of specifying a field, you get the same generic exception plus your custom message (with no indication of the field):
throw new InvalidInputException(com.atlassian.jira.issue.IssueFieldConstants.COMPONENTS, "Summary is incorrect")

In the normal Jira view, you get errors like you would expect, indicating which field is broken and supplying your custom error message to the user.
Bug is open for over 3 years now... I thought Atlassian would care more for the technical quality in detail, but the times are changing obviously.