Users might be unable to save their changes if something is not compatible with CKEditor

As a Wiki user I do not want to waste my time modifying my page only to find out the Editor does not support my content.

Details:

Currently, CKEditor will sometimes raise a banner to tell the user that the page contains source that is not compatible with the Editor. However, this only happens once the user clicks the Source button or Save/Preview. At this point it is too late and the user could have wasted a lot of time. Not only that, the user could make a whole bunch of changes, click the source button and because CKEditor cannot render these changes, they get reverted and all the changes are lost. This is a bad user experience.

Acceptance Criteria
There are two main scenarios we need to solve:

User has clicked the Source button, made some changes that cannot be rendered by CKEditor.

Those changes should not be lost.
The user should be notified the source cannot be rendered in CKEditor.
The user should stay in the source portion of CKEditor.
The user should still be able to save their changes.
User clicks the Edit button on a page which already contains content CKEditor cannot render properly.

The CKEditor edit page should recognize that CKEditor cannot successfully edit the content.
Automatically redirect the user to the Source editor instead.