Fix attr_check messages for ValidationObject
Fix attr_check messages for ValidationObject

Scoring routines expected a list of string.  `attr_check`
ValidationObject case returned a string, rather than a list with one
string element (error case) or no elements (pass case).  This commit
fixes the omission.

ocefpaf/compliance-checker,lukecampbell/compliance-checker,ioos/compliance-checker,aodn/compliance-checker