Improve error message on incorrect machine

[xylar]

Now, when you give an incorrect machine, e.g.:

mpas_analysis -m blah example_e3sm.cfg

you get an error message like:

Traceback (most recent call last):
  File "/home/xylar/code/MPAS-Analysis/improve_error_on_wrong_machine/mpas_analysis/__main__.py", line 799, in main
    config.add_from_package('mache.machines', f'{machine}.cfg')
  File "/home/xylar/mambaforge/envs/mpas_dev/lib/python3.10/site-packages/mpas_tools/config.py", line 87, in add_from_package
    self._add(path, user=False)
  File "/home/xylar/mambaforge/envs/mpas_dev/lib/python3.10/site-packages/mpas_tools/config.py", line 426, in _add
    raise FileNotFoundError(f'Config file does not exist: {filename}')
FileNotFoundError: Config file does not exist: /home/xylar/mambaforge/envs/mpas_dev/lib/python3.10/site-packages/mache/machines/blah.cfg

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/xylar/mambaforge/envs/mpas_dev/bin/mpas_analysis", line 33, in <module>
    sys.exit(load_entry_point('mpas-analysis', 'console_scripts', 'mpas_analysis')())
  File "/home/xylar/code/MPAS-Analysis/improve_error_on_wrong_machine/mpas_analysis/__main__.py", line 809, in main
    raise ValueError(
ValueError: We could not find the machine: blah.
Possible machines are:
  acme1
  andes
  anvil
  badger
  chrysalis
  compy
  cooley
  cori-haswell
  cori-knl
  default
  grizzly

closes #891

[xylar]

@cbegeman, could you review this by inspection (just letting me know if this new output seems intuitive)?

[cbegeman]

@xylar Looks good to me! Thanks!

[xylar]

Thanks for taking a look, @cbegeman!