qapi/introspect.py: improve _tree_to_qlit error message

Trivial; make the error message just a pinch more explicit in case we
trip this by accident in the future.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210216021809.134886-12-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/introspect.py

@@ -126,7 +126,9 @@ def _tree_to_qlit(obj, level=0, dict_value=False):
     elif isinstance(obj, bool):
         ret += 'QLIT_QBOOL(%s)' % ('true' if obj else 'false')
     else:
-        assert False                # not implemented
+        raise NotImplementedError(
+            f"type '{type(obj).__name__}' not implemented"
+        )
     if level > 0:
         ret += ','
     return ret