fix(mflistfile): raise informative error if first budget can't be parsed

[wpbonelli]

If ListBudget failed to parse a value in the first budget table, rather than raise an error, it would just print a message and continue with bad internal state, eventually raising a misleading unpacking error wrapped in a generic Exception. The only mention of the actual problem was the printed line, which easily gets buried under the unhelpful exceptions.

If any values in the first budget table fail to parse, raise ValueError naming the time step, stress period, and invalid line. I left behavior for parse failures in later budget tables unchanged: print a message and fill that time step with NaN. This means the same bad value will raise in the first table but become NaN in any other. This inconsistency isn't good (neither is giving NaN for bad values, IMO, since it makes it impossible to distinguish legitimate NaNs from parse failures), but people may have come to expect and rely on this behavior, so it seems safest to leave a comprehensive fix for 4.x.

Also reword list file parsing messages: "time step x, stress period y" instead of "ts,sp x y", readable names (e.g. "time step length" instead of "tslen"), and report the bad line where available.

This situation will now raise an error like

ValueError: unable to read first budget in list file model.lst: could not parse value for FLOW-JA-FACE (OUT) at time step 1, stress period 1: 'FLOW-JA-FACE =       8.5159-100 ...'

[codecov]

Codecov Report

❌ Patch coverage is 57.69231% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.8%. Comparing base (556c088) to head (b3c55a2).
⚠️ Report is 236 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/mflistfile.py	57.6%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2859      +/-   ##
===========================================
+ Coverage     55.5%    73.8%   +18.2%     
===========================================
  Files          644      659      +15     
  Lines       124135   132404    +8269     
===========================================
+ Hits         68947    97732   +28785     
+ Misses       55188    34672   -20516     

Files with missing lines	Coverage Δ
flopy/utils/mflistfile.py	73.9% <57.6%> (+4.4%)	⬆️

... and 585 files with indirect coverage changes

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