Frontend does not handle error in diffs properly

When diffs.json returns 500 Frontend still shows a loading spinner.

Since most times the page will be scrolled down to the bottom, the flash warning will be missed.

Screen_Shot_2018-10-15_at_11.24.07

Link to reproduce: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7924/diffs

/cc @andr3