This change reverts the significant part of 1dfdb36d

I can't say that I understand why libsignal throws `Error: Invalid signature`
when you try to build a session for your own (sending) device, but given that
messages can only be decrypted once, I guess it isn't really necessary
to encrypt for your own device, since you already have the plaintext.

In addition I've added some error handling so that we can recover
gracefully when session building fails for a subset of devices.