Show users what's wrong when they try to edit an inline with an editor already open

Summary: Fixes T8572. Ideally we would probably just permit this, but clean up the behavior until the day arrives when inline code is actually rewritten.

Test Plan:

• Tried to launch editors in Differential and Diffusion while comments were already open.
• Verified that "Jump to inline" works in both cases.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8572

Differential Revision: https://secure.phabricator.com/D14094