Raise if get_map can't find any maps instead of logging and returning None

[AhmetNSimsek]

Replaces #460

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 46.06%. Comparing base (dc63457) to head (b36aa4b).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
siibra/core/parcellation.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #618      +/-   ##
==========================================
+ Coverage   46.00%   46.06%   +0.05%     
==========================================
  Files          75       75              
  Lines        7232     7240       +8     
==========================================
+ Hits         3327     3335       +8     
  Misses       3905     3905              

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