Modify custom hook response messages and add error logging

This MR makes the following changes to pre-receive, post-receive and update custom hooks:

• All stderr from the executed scripts is logged
• Messages returned when the executed script exits with a failure status code is now comprised of all stdout + stderr. Previously this was stderr or if blank, then stdout. This change aligns with the newly documented behaviour https://gitlab.com/gitlab-org/gitlab-ce/commit/f82380b9df9693e7976b7474233840a469635429#fefd9f80956911f9e14791570ce84e3836703963
• Tests have been written to test the ability for pre-receive and post-receive hook scripts to receive stdin, and update hook scripts to receive arguments.