Fix minor issues in ck2yaml and test suite

[ischoegl]

Fix glitch in test suite / improve exception handling

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

Closes #1666, closes #1656

Illustration

Using the minimal example from #1656 (issue reporting unhelpful exception handling), the output is now

% python -m cantera.ck2yaml --thermo=test/data/surface2-thermo.dat --surface=test/data/surface2.inp

ERROR: Cannot specify a surface mechanism without a gas phase

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/homebrew/Caskroom/miniforge/base/envs/cantera-dev/lib/python3.11/site-packages/cantera/ck2yaml.py", line 2316, in <module>
    main()
  File "/opt/homebrew/Caskroom/miniforge/base/envs/cantera-dev/lib/python3.11/site-packages/cantera/ck2yaml.py", line 2199, in main
    parser, surfaces = Parser.convert_mech(input_file, thermo_file,
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/cantera-dev/lib/python3.11/site-packages/cantera/ck2yaml.py", line 2051, in convert_mech
    raise InputError(msg)
InputError: Cannot specify a surface mechanism without a gas phase
Please check https://cantera.org/stable/userguide/ck2yaml-tutorial.html#debugging-common-errors-in-ck-files
for the correct Chemkin syntax.

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
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