[ENH] Validate the custom rules before storing them

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47277 b456876b-0849-0410-b77d-98878d47e9d5
[ENH] Silently fail when a formula fails executing while processing events, as event handlers should not fail

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47278 b456876b-0849-0410-b77d-98878d47e9d5