Exit gracefully when file not found

[rnjudge]

When I was running the tool I accidentally provided a file to it that did not exist (fat finger typo). The tool gave me a confusing UnboundLocalError message that might be confusing for users not familiar with reading Python tracebacks. Suggestion for the tool to exit gracefully if a non-existent file is supplied by providing a more clear error message.

Currently:

(ternenv) rose@rose-vm:~/ternenv/ntia-conformance-checker/ntia_conformance_checker$ python3 main.py -v --file dne.spdx
ERROR:root:Filename dne.spdx not found.
ERROR:root:Document cannot be parsed: [Errno 2] No such file or directory: 'dne.spdx'
Traceback (most recent call last):
  File "/home/rose/ternenv/ntia-conformance-checker/ntia_conformance_checker/main.py", line 51, in <module>
    main()  # pylint: disable=no-value-for-parameter
  File "/home/rose/ternenv/lib/python3.10/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/rose/ternenv/lib/python3.10/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/home/rose/ternenv/lib/python3.10/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/rose/ternenv/lib/python3.10/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/rose/ternenv/ntia-conformance-checker/ntia_conformance_checker/main.py", line 36, in main
    sbom = sbom_checker.SbomChecker(file)
  File "/home/rose/ternenv/ntia-conformance-checker/ntia_conformance_checker/sbom_checker.py", line 17, in __init__
    self.doc = self.parse_file()
  File "/home/rose/ternenv/ntia-conformance-checker/ntia_conformance_checker/sbom_checker.py", line 42, in parse_file
    return doc
UnboundLocalError: local variable 'doc' referenced before assignment

Could be improved to something like:

(ternenv) rose@rose-vm:~/ternenv/ntia-conformance-checker/ntia_conformance_checker$ python3 main.py -v --file dne.spdx
Warning: file 'dne.spdx' not found.

[jspeed-meyers]

@rnjudge, great call!