rather than having to have the error message passed in.
This is in preparation for not having the dialog pop up straight
away when a message isn't sent so we don't have to keep the error
object knocking around somewhere.
When starting a call, several events are sent and if some devices are unverified, all three will trigger their own UnknownDeviceError. This causes three overlapping, identical UnknownDeviceDialogs.
This change effectively dedupes the dialogs so that only one is shown. This is safe to do because the UDD allows resending of _all_ events that were left unsent.
Fixes https://github.com/vector-im/riot-web/issues/3285
