[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 #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

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

[codecov[bot]]

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).

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

Additional details and impacted files

```diff @@ 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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