Enhanced validation messages for key errors.

[alanisaac]

Fixes #20

This PR:

• Adds another sentence to the key-based errors in the ocsf-validator to help schema developers understand that the errors could be in the source file OR referenced files.
• Removed the "class" part of the message, which would display as OcsfAttr. While this might be useful for debugging the validation library, schema developers would not know what that is referencing unless they checked the code of this library.

Examples:

   ERROR: Unrecognized key `foo` at `attributes.transaction_uid` in events/network/dhcp.json.  Make sure fields in this file and any supporting files such as dictionaries or includes are valid.

   ERROR: Missing required key `caption` at `attributes.transaction_uid` in events/network/dhcp.json.  Make sure required fields in this file and any supporting files such as dictionaries or includes are populated.