jsonb_path_match() checks if jsonb document matches jsonpath query.  Therefore,
jsonpath query should return single boolean.  Currently, if result of jsonpath
is not a single boolean, NULL is returned independently whether silent mode
is on or off.  But that appears to be wrong when silent mode is off.  This
commit makes jsonb_path_match() throw an error in this case.

Author: Nikita Glukhov