Add basic feedback for unauthorized users.

src/backends/github/implementation.js

@@ -31,17 +31,20 @@ export default class GitHub {
   authenticate(state) {
     this.token = state.token;
     this.api = new API({ token: this.token, branch: this.branch, repo: this.repo, api_root: this.api_root });
-    return this.api.user().then(user => (
+    return this.api.user().then(user =>
       this.api.collaborator(user.login).then((status) => {
-        if (status === 404) {
+        if (status === 404 || status === 403) {
           // Unauthorized user
-          throw new Error('You don\'t have publishing access.\nIf you think this is incorrrect, talk to the site administrator.');
+          alert("Sorry, but you don't appear to have publisher access. If you think this is incorrect, talk to the site administrator");
+          throw new Error('Talk to the site administrator to get publishing access.');
+        } else if (status === 204) {
+          // Authorized user
+          user.token = state.token;
+          return user;
         }
-        // Authorized user
-        user.token = state.token;
-        return user;
+        throw new Error('No response from Github, that\'s odd.');
       })
-    ));
+    );
   }
 
   getToken() {