Fix HttpException status line overflow

Previously, error responses generated by HttpException could
overflow the maximum HTTP line length because error messages
were being written into the status line. This change modifies
error handling to always use Content-Type: text/plain for
error responses generated as a result of HttpExceptions (and
other sorts of exceptions) thrown during request processing.

It is important to note that we have to use text/plain since at
the point of handling, no semantic information about the type of
the response is available. So, HttpException should *only* ever
be used for fatal errors that are not part of the defined API
for a given service.