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