Validate config/cache paths

[nsg]

If --cache-path is specified but it's not a valid path, this will come to the users attention first when the cache is written. This could take a while if there are a lot of files. For example:

Got 612 FSM:s. 0 from cache. 612 freshly generated.
FSM generation took: 399.18s
Path collision check took: 145.09s
Writing FSM cache to cache/external.pkl
Traceback (most recent call last):
  File "/home/c/git/orm-rules/env/bin/orm", line 11, in <module>
    sys.exit(main())
  File "/home/c/git/orm-rules/env/lib/python3.7/site-packages/orm/__main__.py", line 78, in main
    cache_path=args.cache_path):
  File "/home/c/git/orm-rules/env/lib/python3.7/site-packages/orm/validator.py", line 48, in validate_rule_files
    return validate_rule_constraints(yml_files, cache_path=cache_path)
  File "/home/c/git/orm-rules/env/lib/python3.7/site-packages/orm/validator.py", line 573, in validate_rule_constraints
    cache_path=cache_path):
  File "/home/c/git/orm-rules/env/lib/python3.7/site-packages/orm/validator.py", line 558, in validate_constraints_rule_collision
    with open(cache_path, 'wb') as fsm_cache_file:
FileNotFoundError: [Errno 2] No such file or directory: 'cache/external.pkl'
make: *** [Makefile:38: ci-output-file] Error 1

It would be better to validate --cache-path as early as possible, for example in __main__.py.

(imported from internal issue)