Improve error message for Conduit path problems

Summary:
A few people in IRC have been having issues here recently. If you misconfigure
the IRC bot, e.g., you get a 200 response back with a bunch of login HTML in it.
This is unhelpful.

Try to detect that a conduit request is going to the wrong path and raise a
concise, explicit error which is comprehensible from the CLI.

Also created a "PlainText" response and moved the IE nosniff header to the base
response object.

Test Plan: As a logged-out user, hit various nonsense with "?__conduit__=true"
in the URI. Got good error messages. Hit nonsense without it, got login screens.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T775

Differential Revision: https://secure.phabricator.com/D1407

src/applications/auth/controller/login/PhabricatorLoginController.php

@@ -30,6 +30,25 @@ class PhabricatorLoginController extends PhabricatorAuthController {
       return id(new AphrontRedirectResponse())->setURI('/');
     }
 
+    if ($request->isConduit()) {
+
+      // A common source of errors in Conduit client configuration is getting
+      // the request path wrong. The client will end up here, so make some
+      // effort to give them a comprehensible error message.
+
+      $request_path = $this->getRequest()->getPath();
+      $conduit_path = '/api/<method>';
+      $example_path = '/api/conduit.ping';
+
+      $message =
+        "ERROR: You are making a Conduit API request to '{$request_path}', ".
+        "but the correct HTTP request path to use in order to access a ".
+        "Conduit method is '{$conduit_path}' (for example, ".
+        "'{$example_path}'). Check your configuration.";
+
+      return id(new AphrontPlainTextResponse())->setContent($message);
+    }
+
     $next_uri = $this->getRequest()->getPath();
     if ($next_uri == '/login/') {
       $next_uri = '/';

src/applications/auth/controller/login/__init__.php

@@ -6,6 +6,7 @@
 
 
 
+phutil_require_module('phabricator', 'aphront/response/plaintext');
 phutil_require_module('phabricator', 'aphront/response/redirect');
 phutil_require_module('phabricator', 'applications/auth/controller/base');
 phutil_require_module('phabricator', 'applications/auth/oauth/provider/base');