long role names fail to sync

Problem/Motivation

Discourse user group names can only be 20 characters long, but Drupal role names can be longer.

If you create a Drupal role with a machine name longer than 20 characters, it won't sync.

Steps to reproduce

Create a Drupal role with a machine name longer than 20 characters.

Assign that role to a user.

Attempt to sync with Discourse.

Proposed resolution

Ideally, check all role machine names to see if truncated versions would conflict, and if not, truncate the long machine names before creating the synced version in Discourse.

Otherwise, at least provide error messages in the Drupal logs and on the Discourse SSO config pages (and maybe on the Status Report).

Thanks for this great module!