Don't notify for bad encrypted messages
Continue to wait for decryption if the decryption process failed.
Because we weren't doing this, the first message in a megolm session
was generally failing to decrypt the first time because the keys often
arrive after the message.
Fixes https://github.com/vector-im/riot-web/issues/6284
This is is small & safe enough that we may want to consider
putting it in 0.14 if we're cutting another RC.
