fix issue #21 by having more informative error message when deleting a custom scorer

[epugh]

Description

I need to delete a custom scorer, but it is tied to other users, cases, or qeuries, but I don't know which. So let me know.

Motivation and Context

Fixeds #21

How Has This Been Tested?

Ran unit tests.

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 updated the documentation accordingly.
• [] I have read the CONTRIBUTING document.
• [] I have added tests to cover my changes.
• All new and existing tests passed.

[epugh]

@worleydl gives thumbs up