Consider telling the user about to-device decryption failures

[richvdh]

If the device couldn't decrypt a to-device event, we just swallow the error, without telling the user about it. Then the user just sees 'the sender didn't send the keys', which is somewhat misleading.

[ylecollen]

The reason is now displayed like the webclient (like Invalid message index or so).

[richvdh]

I don't understand. The webclient also swallows the error.

[richvdh]

Still don't understand.