Mistral returns 500 code on updating nonexistent task

[Sergey Murashov]

Steps to reproduce:
1. Send PUT request on /workbooks/{workbook-name}/executions/{execution-id}/tasks/{nonexistent task id}

Actual result:
We get 500 status code and Details: {"debuginfo": null, "faultcode": "Server", "faultstring": "Failed to create necessary DB objects: 'NoneType' object has no attribute '__getitem__'"}

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/98786

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/98786
Committed: https://git.openstack.org/cgit/stackforge/mistral/commit/?id=f11aa226846c2ca8db4afd93a1f1439ef672cdc4
Submitter: Jenkins
Branch: master

commit f11aa226846c2ca8db4afd93a1f1439ef672cdc4
Author: Nikolay Mahotkin <email address hidden>
Date: Mon Jun 9 17:23:20 2014 +0400

    Fix update nonexistent task

    Closes-bug: #1326284

    Change-Id: I0eeaff7fbf44c975d2aac0a4839ad0f5fecd3276