Raise error if atlases are not collected

[tsalo]

Closes #255.

Changes proposed in this pull request

• Rather than raising a warning if an atlas isn't collected properly, raise an exception.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 12.92%. Comparing base (f6b40c0) to head (39d6216).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
qsirecon/utils/atlases.py	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #256      +/-   ##
==========================================
- Coverage   12.93%   12.92%   -0.02%     
==========================================
  Files          56       56              
  Lines        7899     7907       +8     
  Branches     1071     1072       +1     
==========================================
  Hits         1022     1022              
- Misses       6845     6853       +8     
  Partials       32       32              

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[tsalo]

@mckenziephagen would you mind taking a look at this PR to see if it resolves your issue?