Missing error for unsafe locked transfers

[palango]

Problem Definition

While investigating a mediated transfer from a light client to a python client (with a python client as mediator) I found that the transfer was blocked because the locked transfer was not deemed safe.

https://github.com/raiden-network/raiden/blob/6a98bee0409b2239a12230ce097d3f0bc4b3c64c/raiden/transfer/mediated_transfer/target.py#L120-L136

is_safe_to_wait returns a nice error, but that one is never logged or displayed which makes debugging unnecessary hard.

Also, it's unclear if the mediator has the same check and it just works, or if the mediator is missing this check and should have rejected the payment in the first place.