R1:f81021fa7f2e

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…