Can not append to a rule that was skipped (due to low priority, for example)

[mstemm]

If you create a falco_rules.local.yaml entry that appends to a rule in falco_rules.yaml, but the original rule in falco_rules.yaml was skipped (for example, for being below the configured priority level in falco.yaml) you'll get this misleading error:

$  sudo falco -r /etc/falco/falco_rules.yaml -r /etc/falco/falco_rules.local.yaml > tmp.txt
Fri Dec  8 14:19:29 2017: Falco initialized with configuration file /etc/falco/falco.yaml
Fri Dec  8 14:19:29 2017: Parsed rules from file /etc/falco/falco_rules.yaml
Fri Dec  8 14:19:29 2017: Runtime error: Error loading rules:[string "--..."]:301: Rule Read sensitive file untrusted has 'append' key but no rule by that name already exists. Exiting.

The fix should be to keep track of rules that were skipped, and silently accept additional rules that append to them, but still mark them as skipped.

Thanks to @ross-bragg for pointing out the issue!