Rescue if markup fails, prevent 500, fixes #7455

##### What does this MR do?

This PR renders markup file without markup, if markup fails for some reason.

Why was this MR needed?

Currently, if markup fails for some reason, GitLab displays a 500 error.

What are the relevant issue numbers?

This PR fixes #7455.

It's also related to #7023 and https://gitlab.com/gitlab-org/gitlab-ce/issues/296

/cc @jacobvosmaer