chore: Raise error if Ansys has no attribute Mechanical

[klmcadams]

When this error occurs:

assembly_resolver = Ansys.Mechanical.Embedding.AssemblyResolver
AttributeError: module 'Ansys' has no attribute 'Mechanical'

Raise the following error:

AttributeError: Unable to resolve Mechanical assemblies. Please ensure the following:
    1. Mechanical is installed
    2. A folder with the name "Ansys" does not exist in the same directory as the script being run.

Closes #513

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.30%. Comparing base (bbbd6c3) to head (2eec448).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1062      +/-   ##
==========================================
- Coverage   87.34%   81.30%   -6.04%     
==========================================
  Files          36       40       +4     
  Lines        2362     2755     +393     
==========================================
+ Hits         2063     2240     +177     
- Misses        299      515     +216