Add "slack:failed" task

[parkr]

If the deployment fails, we should allow users to notify slack

[bartzy]

When/where do I need to run slack:failed? How can I tell when a deployment failed?

[parkr]

@bartzy It will depend. We do after "deploy:rollback", "slack:failed".

[bartzy]

But deploy:rollback is executed when any command fails in deploy?

[parkr]

But deploy:rollback is executed when any command fails in deploy?

For us, it is. Capistrano has it built-in, it seems. That said, these sometimes don't work. :/

[bartzy]

Got it, thanks!