Show build failure log in Slack

[arm4b]

When execution failure is posted in #stackstorm Slack

, show latest error as well.

Add it as Slack API attachment/code snippet.
https://api.slack.com/docs/messages/builder

[arm4b]

Seems like it could be better to implement with this mistral feature: https://specs.openstack.org/openstack/mistral-specs/specs/ocata/implemented/publish-on-failure.html publish-on-error.

@m4dcoder Do you know any other ways how to get the stdout/stderr from the latest FAILED action in a mistral workflow?
Do you think anything is possible, instead of modifying every action and adding publish stdout/stderr explicitly?