Gracefully handle when epic tree retrieval times out.
502 Bad Gateway (a timeout) received when trying to retrieve the epic tree.
The error could read something like "the epic tree is too large".
Reproduced on GitLab.com on https://gitlab.com/groups/gitlab-com/gl-security/compliance/-/epics/66:
The timing below suggests it is hitting the puma(?) timeout
This is on canary: 
Edited by charlie ablett