weird crash when failing to get cross-signing keys during verification

For users on a different HS, when trying to verify it sometimes fails. I assume it is failing because of [the federation bug](https://github.com/vector-im/riot-web/issues/12973), but the failure mode is a bit painful. It just does nothing when clicking verify. I would have expected it to launch a to_device verification request, instead of nothing at all, when it can't find cross-signing keys.

In the console I see:
```
2020-04-17T14:50:20.003Z W could not find verifier constructor for method @steve:loynes.modular.im
```
Looks like it is using the username somehow as the verifier method?!

Rageshakes:
- https://github.com/matrix-org/riot-web-rageshakes/issues/2620
- https://github.com/matrix-org/riot-web-rageshakes/issues/2619