better feedback when we can't share keys with a device

[richvdh]

If we can't start an Olm session with a device (eg, because we couldn't claim a one-time key), then we write an error to the console and carry on regardless. We should probably offer the user some options (carry on, try again, etc.)

[poljar]

While this is talking about showing an error on the sending side, we instead implemented showing more helpful errors on the receiving side via m.room_key.withheld messages.

This might have been what we wanted all along, if not, please reopen.