Gracefully handle creation failures of custom resources

[ejholmes]

Fixes #918

See #918 for an explanation of what was causing the issue.

This does a few things:

1. Fixes the issue in Custom CloudFormation resources should gracefully handle stack creation rollbacks #918
2. Pulls out some of the custom resources framework in server/cloudformation, to pkg/cloudformation/customresources.

End result is that, if creating a new resource fails, the correct error will appear in the CloudFormation events, and request to delete it during the rollback will succeed.

[phobologic]

👍