COMMENT is accepted on all keywords and is stored on the event instead of
being passed as keyword data, so it is not part of the item names reported
by opm-common. The validation error listed only the opm-common items, giving
no hint that COMMENT is allowed when a user mistyped it as COMMENTS.
Append COMMENT to the list of valid item names in the error message. Only the
diagnostic is affected, COMMENT is still handled as event metadata.