Validate the directives that appear at a vertex field.

def validate_vertex_directives(directives):
    """Validate the directives that appear at a vertex field."""
    for directive_name in six.iterkeys(directives):
        if directive_name in PROPERTY_ONLY_DIRECTIVES:
            raise GraphQLCompilationError(
                u'Found property-only directive {} set on vertex.'.format(directive_name))