trust subscriber's safety number on failed message broadcast attempt

Context

Failed send looks as described here: https://gitlab.com/thefinn93/signald/issues/4

Ie: after attempting to broadcast a message to a subscriber with an new and untrusted safety number, signald will send a message to its unix socket that looks like this:

{
  "type":"unexpected_error",
  "data":{
    "msg_number":0,
    "error":true,
    "request":{
      "type":"send",
      "username":"+1<REDACTED>",
      "messageBody":"[TEST_WELCOME]\nHi",
      "recipientNumber":"+1<REDACTED>",
      "attachments":[],
      "expiresInSeconds": 0
    }
}