Suppress RuntimeException in ExecModelLambdaPostProcessor

When we hit an issue in ExecModelLambdaPostProcessor, we want to silently fall back to non-externalized lambda (don't want to break a build). Currently ExecModelLambdaPostProcessor throws RuntimeException only in unusual cases but still it's better to log and return rather than throwing RuntimeException.

Btw, we can detect non-externalized lambda with option "drools.check.nonExternalisedLambda" (DROOLS-5346).