Return user who already claimed a submission

[TimJentzsch]

Relevant issue: GrafeasGroup/tor#187, Closes #184

Description:

When a submission is claimed and then tried to claim again, the user who already claimed the submission is returned along with the error code. u/tor can use this to determine if it was the same person or another person and adjust the message accordingly.

Checklist:

• [X] Code Quality
• [X] Pep-8
• [X] Tests (if applicable)
• [X] Success Criteria Met
• [X] Inline Documentation
• [ ] Wiki Documentation (if applicable)