Exit with OCF_ERR_INSTALLED on manifest or env var lookup failure

The agent relies on the pod manifest existing. If it's not found or if
the requested environment variable is missing, the resource agent should
exit with $OCF_ERR_INSTALLED rather than $OCF_ERR_GENERIC or propagating
the jq exit code.

A comment was added to clarify the seemingly redundant file existence
check, which is kept for robustness.