docs: def addError(self, test, err, capt=None):
          """Add error output to Xunit report.
          """
          exc_type, exc_val, tb = err
          tb = ''.join(traceback.format_exception(
              exc_type,
              exc_val if isinstance(exc_val, exc_type) else exc_type(exc_val),
              tb
          ))
          name = id_split(test.id())
          group = self.report_data[name[0]]
          if issubclass(err[0], SkipTest):
              type = 'skipped'
              self.stats['skipped'] += 1
              group.stats['skipped'] += 1
          else:
              type = 'error'
              self.stats['errors'] += 1
              group.stats['errors'] += 1
          group.tests.append({
              'name': name[-1],
              'failed': True,
              'type': type,
              'errtype': nice_classname(err[0]),
              'message': exc_message(err),
              'tb': tb,
          })