[hf] safely destructure identity failure errors

* BUG SYMPTOM: some send response handlers error when they try to
  destructure a null `data` field
* CAUSE: the `{identityFailure}` destructuring assignment unsafely
  assumes that all `send_message` values will have a `data` field on
  which we can call `data[0]`
* FIX: don't assume anything about the shape of the `data`
  field. extract `identityFailure` from it (if it is there) with a
  lodash `get` call to the full path to the field we are trying
  to (conditionally) extract

app/signal/callbacks.js

@@ -102,8 +102,7 @@ const handle = message => {
 const _handleSendResponse = ({ message, state }) => {
   delete registry[`${messageTypes.SEND}-${message.id}`]
 
-  const { identityFailure } = get(message, 'data[0]')
-  if (identityFailure) {
+  if (get(message, 'data.0.identityFailure')) {
     return _updateFingerprint(message, state)
   }