check_project_configuration() should be more robust

[vladak]

Production instance had this in mirror.yml:

projects:
  foo:
    # ha

which produced:

Traceback (most recent call last):
  File "/opengrok/dist/bin/venv/bin/opengrok-mirror", line 11, in <module>
    load_entry_point('opengrok-tools==1.3.1', 'console_scripts', 'opengrok-mirror')()
  File "/opengrok/dist/bin/venv/lib/python3.7/site-packages/opengrok_tools/mirror.py", line 175, in main
    with lock.acquire(timeout=0):
  File "/opengrok/dist/bin/venv/lib/python3.7/site-packages/filelock.py", line 271, in acquire
    self._acquire()
  File "/opengrok/dist/bin/venv/lib/python3.7/site-packages/filelock.py", line 384, in _acquire
    fd = os.open(self._lock_file, open_mode)

This change fixes that.