Adds specified item check result to this cart check result object.

Returns formatted error message when check has failed, meaning that errors from each item are on its own line using end-of-line char. Returns empty string if check passed.

Returns HTML formatted error message when check has failed, meaning that errors from each item are on its own line using [br /] HTML tag. Returns empty string if check passed.

Returns check result formatted as string. Message consists of common error message followed by individual item error messages separated using specified separators. Empty string is returned when no errors found.