Better warning messages for open vocabs and relationship types

[rpiazza]

Before you were on the working call today, we discussed this issue, which Sean has said he has been asked about.

We thought maybe there could be more "gentle" warning for such messages. Here are some ideas off the top of my head:

's value is permitted, but not in

Relationship type is permitted, but not explicitly defined in the specification

[rpiazza]

Change the warning message at https://github.com/oasis-open/cti-stix-validator/blob/master/stix2validator/v21/shoulds.py#L539 to say:

'%s' is permitted, but not a suggested relationship type for '%s' objects

and at https://github.com/oasis-open/cti-stix-validator/blob/master/stix2validator/v21/shoulds.py#L390 to say:

"The value contained in %s is permitted, but is not in the %s-ov vocabulary."

[rpiazza]

use this file to test:

malware-indicator-for-file-hash.json