For exception management, log the exception stack trace as a structured log when sentry is disabled
This issue arose through this discussion: https://gitlab.com/gitlab-org/gitlab-ce/issues/48747#note_173313093
Unlike GitLab.com, most GitLab instances do not run Sentry. When a 5xx error occurs, the exception reporting is a no-op.
In these circumstances, we should log the full stack trace of the error, along with the other details that we send to sentry to a structured log.
This will allow us to better investigate and analyse errors in environments without Sentry.
This is dependent on !12 (merged) and !13 (closed)