Implement the "error" VCL keyword:

Implement the "error" VCL keyword:

 - add fields to struct sess where VRT_error can store the error code and
   message
 - modify cnt_error() to pass these fields to RES_Error(), then clear them
 - modify RES_Error() (and the entire chain) to accept a third argument
   giving an explanation of the error.
 - have RES_Error() reset the worker before writing the error document, to
   make sure wfd is set.

fixes: #4

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@823 d4fa192b-c00b-0410-8231-f00ffab90ce4