TST: Test dMRI base model class exceptions and attributes

[jhlegarreta]

Test dMRI base model class exceptions and attributes.

Take advantage of the commit to define the exception messages as global variables so that the exceptions raised by the class instantiation get checked.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.29%. Comparing base (a2bae01) to head (6a3b553).
⚠️ Report is 30 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   78.65%   79.29%   +0.64%     
==========================================
  Files          24       24              
  Lines        1443     1449       +6     
  Branches      166      166              
==========================================
+ Hits         1135     1149      +14     
+ Misses        237      233       -4     
+ Partials       71       67       -4     

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