To make easier to filter and display errors on the UI, the validators
have been modified to return the message and line number separately. The
return format for validators is now:
(is_valid, [(error_message_string, error_line_number)])
Also the XSD based validators were returning only the last error found on
the document, instead of iterating the whole error log. Harvesters should
create a harvest object error for each of this validation errors.
Tests have been adapted to these changes.
e12e38cab0