Log first 200 bytes of response when MIME type is unexpected

[r-n-o]

This issue stems from a comment from @james-callahan in #13:

You may want to log e.g. the first 200 bytes of the response as debug level logging?
It can be useful for weirder connectivity problems e.g. the user is behind a HTTP(S) proxy; or on captive portal wifi that starts returning "you must be logged in" or something.

This is a good first issue to take on; if nobody gets to it I'll probably tackle it soon!