add username field to untrusted identity error messages

Context

see: https://gitlab.com/thefinn93/signald/issues/4

• commit 5a9a7d8b introduced clean error messaging for instances in which messages fail to send because the recipient's safety number has changed
• however, this message only exposes the number of the recipient, not the number of the sender (ie: the username of the relevant running signald process)
• as noted here, the omission of this field makes it more difficult to retrust safety numbers in services that are operating many usernames (such as signalboost, which i maintain)

Changes

• make a small tweak to expose the username of the sender of the failed message to the untrusted_identity error message
• also include the request body in the error message to make it easier to resend messages that failed to send because of changes safety numbers