Implemented /auth/error path

frontend/scenes/ErrorAuth/ErrorAuth.js

@@ -0,0 +1,23 @@
+import React from 'react';
+import { Link } from 'react-router';
+
+import Layout from 'components/Layout';
+import CenteredContent from 'components/CenteredContent';
+
+class ErrorAuth extends React.Component {
+  render() {
+    return (
+      <Layout
+        titleText="Not Found"
+      >
+        <CenteredContent>
+          <h1>Authentication failed</h1>
+
+          <p>We were unable to log you in. <Link to="/">Please try again.</Link></p>
+        </CenteredContent>
+      </Layout>
+    );
+  }
+}
+
+export default ErrorAuth;

frontend/scenes/ErrorAuth/index.js

@@ -0,0 +1,2 @@
+import ErrorAuth from './ErrorAuth';
+export default ErrorAuth;

frontend/scenes/SlackAuth/SlackAuth.js

@@ -12,7 +12,17 @@ class SlackAuth extends React.Component {
   }
 
   componentDidMount = async () => {
-    const { code, state } = this.props.location.query;
+    const { error, code, state } = this.props.location.query;
+
+    if (error) {
+      if (error === 'access_denied') {
+        // User selected "Deny" access on Slack OAuth
+        browserHistory.push('/');
+      } else {
+        browserHistory.push('/auth/error');
+      }
+      return;
+    }
 
     if (this.props.route.apiPath) {
       try {