Catch exceptions when encrypting events
If an exception was thrown by the encryption process, the event would be
queued, but the exception would not be handled. This meant that the event got
stuck as a grey 'sending' event in the UI.
Fixing this correctly is slightly more complex than just handling the exception
correctly. A naive approach would mean that the event would be shown as a red
'unsent' message, and clicking 'resend' would then send the message *in the
clear*. Hence, move the encryption to _sendEvent, where it will be called again
when the event is resent.
