Verifies this filter set can process the result data.

def Validate(self):
    """Verifies this filter set can process the result data."""
    # All filters can handle the input type.
    bad_filters = []
    for f in self.filters:
      try:
        f.Validate()
      except DefinitionError as e:
        bad_filters.append("%s: %s" % (f.expression, e))
    if bad_filters:
      raise DefinitionError(
          "Filters with invalid expressions: %s" % ", ".join(bad_filters))