Code review comment for lp://staging/~james-w/lazr.restfulclient/reject-non-json-responses

bug shows what happens if you get an unexpected content-type
from the server. This change is an attempt to at least give it
a clearer error message and make it easier to debug.

It firstly uses the content-type returned by the response to
interpret it, rather than hardcoding application/json. It's
conceivable that wadllib knows how to deal with such a content-type,
so it's better to let it find the error.

The second change catches wadllib's error and throws an error
that explains the situation, so you would get something like

   https://api.launchpad.net/beta/ubuntu did not return a usable
   content type: This resource doesn't define a representation for
   text/html.

with a .content attribute you can print as usual to find out what
you got instead.

I would appreciate some guidance in how to write tests for this
change.

Thanks,

James