circuitbuilder: return reasons on circuit build errors

            return None, "Can not build a circuit, invalid path."

        path = self._normalize_path(path)

        if path is None:

            return None

            return None, "Can not build a circuit, no path."

        num_missing = len(['foo' for r in path if not r])

        insert_relays = self._random_sample_relays(

            num_missing, [r for r in path if r is not None])

        if insert_relays is None:

            path = ','.join([r.nickname if r else str(None) for r in path])

            log.warning(

                'Problem building a circuit to satisfy %s with available '

                'relays in the network', path)

            return None

            return None, "Can not build a circuit with the current relays."

        assert len(insert_relays) == num_missing

        path = [r.fingerprint if r else insert_relays.pop().fingerprint

                for r in path]