ANW-1341: Display user-friendly message when trying to delete an agent linked to a user

[andrew-morrison]

Replacement for #3030 submitted from a fresh fork, which doesn't have commits from GitHub Actions automatically run on the other fork.

Description

Check whether an agent is linked to a user before deleting, rather than letting it fail if it violates a foreign key constraint.

Related JIRA Ticket or GitHub Issue

https://archivesspace.atlassian.net/browse/ANW-1341

How Has This Been Tested?

1. Log in to staff interface.
2. Find an agent which corresponds to a user.
3. Click the delete button.

It correctly fails to delete, as before, but the message displayed is, in English...

Record cannot be deleted as it is linked to a user

...rather than...

Record deletion failed: Java::JavaSql::SQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails (archivesspace.user, CONSTRAINT user_ibfk_1 FOREIGN KEY (agent_record_id) REFERENCES agent_person (id))`

Attempting to delete a user's agent via the API return a 409 response:

{'error': 'linked_to_user'}

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have authority to submit this code.
• I have added tests to cover my changes.
• All new and existing tests passed.

[andrew-morrison]

The test failures appear to be unrelated to this pull request.