Fix gitlab-shell panic when log file not writable

Stan Hurequested to merge
sh-backport-gitlab-shell-fix-13-9 into 13-9-stable-ee

If the log directory (e.g. /var/log/gitlab/gitlab-shell) is not writable, gitlab-shell will panic and fail to serve SSH connections.

This backports gitlab-shell!453 (merged) into the 13-9-stable-ee branch.

Relates to gitlab-shell#513 (closed)

Edited by Stan Hu

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