TaskService.complete() throws NullPointerException without a meaningful message when a RuntimeManager with the deploymentId is not registered

Description of problem:

Platform BZ for https://issues.jboss.org/browse/JBPM-4852

Steps to Reproduce:
1. See test case in the PR (https://github.com/droolsjbpm/jbpm/pull/350)

Actual results:

Throws NullPointerException

Expected results:

Throws RuntimeException with a message like "No RuntimeManager registered with identifier: id-1"