Handle error if commit file not present

[aj3sh]

commitlint throws python stack error if commit message file not present.

Steps to Replicate:

commitlint --file /non-existent/file

Observed Behaviour:

Traceback (most recent call last):
  File "/Users/aj3sh/.venv/test/bin/commitlint", line 8, in <module>
    sys.exit(main())
  File "/Users/aj3sh/projects/python/commitlint/src/commitlint/cli.py", line 210, in main
    commit_message = _get_commit_message_from_file(args.file)
  File "/Users/aj3sh/projects/python/commitlint/src/commitlint/cli.py", line 139, in _get_commit_message_from_file
    with open(abs_filepath, encoding="utf-8") as commit_message_file:
FileNotFoundError: [Errno 2] No such file or directory: '/non-existent/file'

Expected Behaviour:
A valid message should be shown in this case.