vcard_validators: Factored out checks for presence/absence of params
 vcard_validators: Factored out expected lengths of values/subvalues
vcard_validators: Update for latest changes from master

Also, fixed a few whitespace issues revealed by the PEP8 portion of `make test`.
Include filename in error messages
Format code
Validate `URL` property
Refactor all error-related code into vcard_errors
vcard_errors: Rename `_show_warning` to `show_warning`