[thermo] Improve solveCubic exception handling

[ischoegl]

Changes proposed in this pull request

• Convert writelog messages for MixtureFugacity::solveCubic to exception (no root found) and warning (merged roots) respectively.
• Band-aid until permanent fix for MixtureFugacityTP::solveCubic fails for certain temperatures and pressures #1699 is found (as recommended on UG post)

If applicable, fill in the issue number this pull request is fixing

Band-aid for #1699

The example from #1699 now fails with:

CanteraError:
*******************************************************************************
CanteraError thrown by MixtureFugacityTP::solveCubic:
root failed to converge for T = 250.45022511255627, p = 30717382.242601085 with V = -8038.658839830863
*******************************************************************************

which allows for improved exception handling (rather than checking for invalid densities).

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.25%. Comparing base (1b48991) to head (a8dde95).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/thermo/MixtureFugacityTP.cpp	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1819   +/-   ##
=======================================
  Coverage   73.24%   73.25%           
=======================================
  Files         383      383           
  Lines       54625    54624    -1     
  Branches     9103     9103           
=======================================
+ Hits        40012    40015    +3     
+ Misses      11606    11603    -3     
+ Partials     3007     3006    -1     

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[speth]

Thanks, @ischoegl!