Added an error page to mist

If there's an error that occurs while proxying requests, mist will now send
an error report page to the client. It only shows the message of an error
but it should help with spotting configuration issues.

It's also not very pretty, but it works.
Fixed incorrect response code in error response

All error responses had a HTTP status code of 200 when they were supposed to use the specified code in the parameters.

This is now fixed.
Allows persistant connections through the proxy

By default the connection will persist on, with a 5 second timeout to
match Apache's timeout value, and much for the same reason, to keep load
down.

It tries to watch out for ErrPersistEOF and tries to handle appropriately,
but I've not witnessed any of these appearing, so I'm not sure it will do.