Mistral doesn't raise unpredictable errors

[Nikolay Makhotkin]

Mistral doesn't raise unpredictable errors if error is thrown from engine via oslo.messaging to API.

When engine raises some error, oslo.messaging wraps it in messaging.rpc.client.RemoteError and transfers back to API. This transferred info contains only error class name and error text. Then we need to unwrap RemoteError and raise the real exception. A bit earlier we
assumed that all the errors should be at least MistralException (from module mistral.exceptions) but it seems engine can raise anything error, say, one of builtin exceptions - e.g. AttributeError. Need to see what error is come, check for it existing and raise it.

Mistral tries to find given wrapped error in exceptions package but it is not in it, so mistral in this case raises 'Attribute error: No such attribute <exception class>'

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/225587

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/225587
Committed: https://git.openstack.org/cgit/openstack/mistral/commit/?id=864e01b284debd189f824f8ba26e584868cca3f0
Submitter: Jenkins
Branch: master

commit 864e01b284debd189f824f8ba26e584868cca3f0
Author: Nikolay Mahotkin <email address hidden>
Date: Mon Sep 21 11:57:21 2015 +0300

    Fixing searching errors in mistral.exceptions

     * Keep the original exception and reraise

    Closes-Bug: #1460669

    Change-Id: Ieb3e55b4ceb4051e9f1c2a82b2228b34d3195bda