Add in check for field existence when applying outbound rules

[tylertreat-wf]

RestRules are validated on server start, but if the user has an error in their rest handler code, i.e. they return a different type of resource than their rules specify, go-rest will panic if the field doesn't exist. Consider adding a check to ensure the field exists so a more meaningful error message can be provided.

https://github.com/Workiva/go-rest/blob/master/rest/rule.go#L461