Reporting invalid configuration extensions

[t00sa]

If an invalid configuration file - e.g. a .txt file rather than a .py file - is passed to the CLI using the -configuration argument, the command reports that it is unable to find the file:

linux> touch empty.txt
linux> python -m stylist -configuration empty.txt check_nan_inf_mod.f90
Traceback (most recent call last):
  File "/net/project/ukmo/scitools/opt_scitools/conda/deployments/default-2023_05_04/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/net/project/ukmo/scitools/opt_scitools/conda/deployments/default-2023_05_04/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/var/tmp/persistent/stylist/source/stylist/__main__.py", line 203, in <module>
    main()
  File "/var/tmp/persistent/stylist/source/stylist/__main__.py", line 189, in main
    configuration = __configure(arguments.configuration)
  File "/var/tmp/persistent/stylist/source/stylist/__main__.py", line 111, in __configure
    configuration = load_configuration(project_file)
  File "/var/tmp/persistent/stylist/source/stylist/configuration.py", line 53, in load_configuration
    raise StylistException(message)
stylist.StylistException: Unable to find configuration file: empty.txt
linux> 

It would be better if the error indicated that the the file was invalid rather than reporting that it could not be found. This was spotted during testing for PR 123.