Improve error messaging when Confluence space key already exists

Summary

Jira Service Management's link to Confluence allows users who have the appropriate permission to create a Space from within a service project. If the key that a user provides already exists, users are met with the error:

We couldn't create a space for you. Confluence might be down, or you might not have permission to create a space. Learn about permissions.

It's not clear from the error message that the issue is caused by an existing space key. Improving the error message would be helpful to alert end users of the issue.