Workflow Verification

Created by: jjenthought

Required by https://github.com/force-h2020/force-wfmanager/pull/194 as some of the error reporting things are now in verifier.py. Changes the VerifierError object to have a local & global error message, which can be the same. The global message should be something useful when looking at the workflow as a whole, and the local one useful when the object causing the error is selected/ being edited.