Need to handle error during YAQL evalution if non-defined variable was called

[Anastasia Kuznetsova]

Steps to reproduce:
1. Create workflow definition
For example:
---
version: "2.0"

wf_main:

  tasks:
    invoke_wf:
      with-items:
       - i in [1, 2, 3]
      workflow: wf_work var="<% $.i + $.j %>"
      publish:
        res: <% $.invoke_wf %>
2. Execute "mistral workflow-create <def.yaml>"
3. Try to execute this workflow

Observed result:
unsupported operand type(s) for +: 'int' and 'NoneType'
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 142, in _dispatch_and_reply
    executor_callback))
  File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 186, in _dispatch
    executor_callback)
  File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 130, in _do_dispatch
    result = func(ctxt, **new_args)
  File "/home/ubuntu/mistral/mistral/engine/rpc.py", line 96, in start_workflow
    **params
  File "/home/ubuntu/mistral/mistral/utils/__init__.py", line 104, in _logged
    return func(*args, **kw)
  File "/home/ubuntu/mistral/mistral/engine/default_engine.py", line 87, in start_workflow
    raise e
TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

Expected result:
User friendly error: "Variable 'foo' was undefined"

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/180953
Committed: https://git.openstack.org/cgit/stackforge/mistral/commit/?id=d55ed99b5051a1e076336780e386a4eb23c56eeb
Submitter: Jenkins
Branch: master

commit d55ed99b5051a1e076336780e386a4eb23c56eeb
Author: Nikolay Mahotkin <email address hidden>
Date: Thu May 7 14:42:33 2015 +0300

    Fixing YAQL related errors

     * Now Mistral raises YaqlEvaluationException if
       something was wrong with evaluating YAQL.

    Closes-Bug: #1449505

    Change-Id: Ic6a1592b8424ebdbaaca698b9a894d589def97d2