fix(mfdataplist): raise proper MFDataException on wrong col count

[wpbonelli]

Fix a minor issue mentioned in #2758. When a dataframe passed to set_data() has the wrong column count, the intended error would not be raised, instead you'd get a KeyError.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (725128b).
⚠️ Report is 169 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2766      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132686    +8551     
===========================================
+ Hits         68947    96269   +27322     
+ Misses       55188    36417   -18771     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfdataplist.py	71.9% <ø> (-6.9%)	⬇️

... and 579 files with indirect coverage changes

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