[16.0] Broken 'locked document' panel in Exp. Mgr.

Port CMS-15721 to release/16.0

When a document is locked by another user and it's opened in the Exp. Mgr. visual editor, the red feedback panel no longer shows. 

It should be a response to a 404 on the /editable request that looks like this on 15.4:
https://issues.onehippo.com/secure/attachment/89091/89091_CMS-15721.png

The response of the 403 is OTHER_HOLDER:

{
    "reason": "OTHER_HOLDER",
    "params": {
        "publicationState": "changed",
        "displayName": "Introduction speech",
        "userName": "Jeroen Hoffman",
        "userId": "jhoffman-admin"
    }
}

Reproduction

• Open a document for editing, e.g. https://hap-15-5.eng02.bloomreach.cloud/cms/content/path/content/documents/hap/events/2015/08/introduction-speech

• In another browser, log in as different user, go to the same document
• View it in channel (View menu)
• Open in visual editor

Expected: red feedback panel
Actual: editor remains white, seems to be still loading