Editor error messages aren't stacked

Summary

When error messages appear in the editor, they aren't combined or replaced. If the same error message occurs, it keeps getting added to the list of messages.

Steps

1. Open an editor and leave the editor open.
2. Kill Confluence/delete the xsrf token from the page/Kill the user session

What happened

Error messages keep being added to the screen.

What should have happened

There should only be one message of the same kind.

Notes

This could happen in a number of scenarios. Ideally, we would be able to improve the general error-handling experience.