Remove errors from the cache when they are shown to the user (#1392)

There's really no reason to retry encryption errors again if they've
already been made user-visible in a conversation.

Also, refactor e->error in background.js onError(), since both e and ev
in this method made it too easy to make a mistake.

js/libtextsecure.js

@@ -38631,6 +38631,7 @@ MessageReceiver.prototype.extend({
             var ev = new Event('error');
             ev.error = error;
             ev.proto = envelope;
+            ev.confirm = this.removeFromCache.bind(this, envelope);
 
             var returnError = function() {
                 return Promise.reject(error);