Add amOwner() and amWriter(). Reject sendMessage() if we lack write access.

static/cosmopolite.js

@@ -413,7 +413,7 @@ Cosmopolite.prototype.sendMessage = function(subject, message) {
         (subject['readable_only_by'] || subject['writable_only_by'])) {
       console.log(this.loggingPrefix_(),
                   'local subjects can\'t have ACLs:', subject);
-      reject();
+      reject(new Error('Local subject with ACL'));
       return;
     }
 

static/hogfather.js

@@ -109,6 +109,22 @@ hogfather.PublicChat.prototype.getID = function() {
 };
 
 
+/**
+ * @return {boolean}
+ */
+hogfather.PublicChat.prototype.amOwner = function() {
+  return this.owners_.indexOf(this.cosmo_.currentProfile()) >= 0;
+};
+
+
+/**
+ * @return {boolean}
+ */
+hogfather.PublicChat.prototype.amWriter = function() {
+  return this.writers_.indexOf(this.cosmo_.currentProfile()) >= 0;
+};
+
+
 /**
  * @private
  * @param {Cosmopolite.typeMessage} message
@@ -162,10 +178,20 @@ hogfather.PublicChat.prototype.getMessages = function() {
  * @return {Promise}
  */
 hogfather.PublicChat.prototype.sendMessage = function(message) {
-  return this.cosmo_.sendMessage(this.subject_, {
+  return new Promise(function(resolve, reject) {
+    if (!this.amWriter()) {
+      reject(new Error('Write access denied'));
+      return;
+    }
+    this.cosmo_.sendMessage(this.subject_, {
       type: 'message',
       message: message,
+    }).then(function(msg) {
+      resolve(msg);
+    }).catch(function(err) {
+      reject(err);
     });
+  }.bind(this));
 };