refactor(get_cell_vertices): raise helpful messages, improve docs, add tests

[mwtoews]

This is a general refactor of get_cell_vertices() to raise helpful exception messages, improve the docstrings, and add some missing tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.3%. Comparing base (b142b08) to head (df85678).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2125   +/-   ##
=======================================
  Coverage     71.3%   71.3%           
=======================================
  Files          259     259           
  Lines        59511   59520    +9     
=======================================
+ Hits         42479   42492   +13     
+ Misses       17032   17028    -4     

Files	Coverage Δ
flopy/discretization/structuredgrid.py	47.5% <100.0%> (+0.9%)	⬆️

... and 1 file with indirect coverage changes