Fix decrypt error handling

This rejection-handler was resolving its promise rather than allowing
the rejection to bubble up.

js/libtextsecure.js

@@ -39595,6 +39595,7 @@ function generateKeys(count, progressCallback) {
                 ev.error = error;
                 ev.proto = envelope;
                 this.target.dispatchEvent(ev);
+                throw error; // reject this promise
             }.bind(this));
         },
         handleSentMessage: function(destination, timestamp, message) {