Simplify the generation of error responses.

Simplify the generation of error responses.

Don't take a detour around cache_response.c, go directly to
cache_synthetic.c.

Don't wast time and energy mucking about with pseudo-objects,
just synthesize the error body directly in the workspace and
send it from there.

Close the session after sending error response, freeing up
resources sounds like a good strategy. (XXX: for futher study).

This might be related to #167

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2266 d4fa192b-c00b-0410-8231-f00ffab90ce4