Rule Evaluate Panics are not Handled

[zacharyzundel]

Currently if "e.WhenScope.Evaluate" results in a panic (for example if a rule is written that calls a custom function with the incorrect number of arguments) then the panic is not caught.

This can result in the engine completely failing to process all documents.

I submitted #374 to help address it.

This changes means that a malformed rule can be simply skipped based on the status of g.ReturnErrOnFailedRuleEvaluation in Engine.go.