Fix review_labels

[rwood-97]

Summary

Fixes #400

Describe your changes

Previous code allowed you enter a new label when reviewing labels but would give you an error (even though it relabelled the patch fine). Now we give an error and only allow user to enter one of the existing labels when re-labelling/reviewing their annotations.

Checklist before assigning a reviewer (update as needed)

• Self-review code
• Ensure submission passes current tests
• Add tests
• Update relevant docs

Reviewer checklist

Please add anything you want reviewers to specifically focus/comment on.

• Everything looks ok?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 62.45%. Comparing base (3d0aae0) to head (ca42977).

Files	Patch %	Lines
mapreader/classify/load_annotations.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
- Coverage   62.48%   62.45%   -0.03%     
==========================================
  Files          39       39              
  Lines        6771     6774       +3     
==========================================
  Hits         4231     4231              
- Misses       2540     2543       +3     

Flag	Coverage Δ
unittests	62.45% <0.00%> (-0.03%)	⬇️

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