fix: catch io.copy errors when serving content

What does this MR do?

While investigating https://gitlab.com/gitlab-org/gitlab-pages/-/issues/690 I have found a bunch of errors being ignored around the codebase. This is problematic, especially this case: the error is ignored and a 200 OK status is returned.

This is vendored code so there's a redundant if to reduce the diff with upstream code.

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines