Avoid crash of fatbuildrctl when spec not found

[rezib]

fatbuildrctl crashes when RPM spec file is not found:

$ fatbuildrctl --uri https://build.rackslab.io/devs build -a firehpc -d el8 --sources 1.1.0a1.dev1285@. 
Traceback (most recent call last):
  File "/usr/bin/fatbuildrctl", line 33, in <module>
    sys.exit(load_entry_point('Fatbuildr==2.1.0.dev131', 'console_scripts', 'fatbuildrctl')())
  File "/usr/lib/python3/dist-packages/fatbuildr/cli/__init__.py", line 34, in run
    cls()
  File "/usr/lib/python3/dist-packages/fatbuildr/cli/fatbuildrctl.py", line 574, in __init__
    args.func(args)
  File "/usr/lib/python3/dist-packages/fatbuildr/cli/fatbuildrctl.py", line 1152, in _run_build
    ).architecture_dependent
  File "/usr/lib/python3/dist-packages/fatbuildr/artifact.py", line 280, in architecture_dependent
    with open(check_file, 'r') as fh:
FileNotFoundError: [Errno 2] No such file or directory: '/home/remi/Documents/git/products/hpck.it/firehpc/rpm/firehpc.spec'

This shouldn't happen.