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/__phutil_library_map__.php

@@ -58,6 +58,7 @@ phutil_register_library_map(array(
     'AphrontPageView' => 'view/page/base',
     'AphrontPagerView' => 'view/control/pager',
     'AphrontPanelView' => 'view/layout/panel',
+    'AphrontPlainTextResponse' => 'aphront/response/plaintext',
     'AphrontQueryAccessDeniedException' => 'storage/exception/accessdenied',
     'AphrontQueryConnectionException' => 'storage/exception/connection',
     'AphrontQueryConnectionLostException' => 'storage/exception/connectionlost',
@@ -830,6 +831,7 @@ phutil_register_library_map(array(
     'AphrontPageView' => 'AphrontView',
     'AphrontPagerView' => 'AphrontView',
     'AphrontPanelView' => 'AphrontView',
+    'AphrontPlainTextResponse' => 'AphrontResponse',
     'AphrontQueryAccessDeniedException' => 'AphrontQueryRecoverableException',
     'AphrontQueryConnectionException' => 'AphrontQueryException',
     'AphrontQueryConnectionLostException' => 'AphrontQueryRecoverableException',