feature and fix for 500 on group missing

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/3352

And maybe https://gitlab.com/gitlab-org/gitlab-ce/issues/5600 (looking into this one now, as it may require a different fix).

With the fix it will return a 404 if a group is not found. Added relevant feature.

Title changed from feature and fix for 500 on group missing to WIP: feature and fix for 500 on group missing

Reassigned to @jameslopez

Added 1 commit:

• d6a18613 - fix for failing spec

mentioned in issue #5600 (closed)

Title changed from WIP: feature and fix for 500 on group missing to feature and fix for 500 on group missing

Reassigned to @rspeicher

@rspeicher feel free to code review - it's a small change

Added 1 commit:

• eef9c9d4 - update fix based on feedback

Added 1 commit:

• 5eb936dc - moved and refactored feature based on feedback. Also updated CHANGELOG

Added 43 commits:

• 5eb936dc...5e0ee54c - 42 commits from branch gitlab-org:master
• afde800b - Merge branch 'master' of gitlab.com:gitlab-org/gitlab-ce into fix/unexistent-group-500

Status changed to merged

mentioned in commit 56538f9c

@jameslopez I marked it as fixing both of the mentioned issues. One was already closed, however.

Mentioned in commit pfjason/gitlab-ce@55fcd3db