Add http_error callback

Allows user code to display its own error status responses or just log
the errors. Is called even in cases where begin_request is not called.
Revert "Add http_error callback"

This reverts commit 27fe9c8f01679a1a093f901860e5b2c95c9673c5.
http_error callback

Allow user to display status code errors
Merge pull request #92 from jmucchiello/send_http_error

Send http error
Merge pull request #3 from valenok/master

sync