Handle no responseJSON coming back from POSTs/xhrs

[bobrippling]

What does this PR aim to accomplish?:

This resolves issues with frontend JS when the server fails to send a responseJSON reply, leaving the frontend hanging in a "loading" state.

This also resolve these console errors:

Uncaught TypeError: xhr.responseJSON is undefined
    <anonymous> https://<host>/admin/scripts/js/login.js?v=0:171
    jQuery 6
    <anonymous> https://<host>/admin/scripts/js/login.js?v=0:163
    jQuery 13

Uncaught TypeError: right-hand side of 'in' should be an object, got undefined
    wrongPassword https://<host>/admin/scripts/js/login.js?v=0:33
    doLogin https://<host>/admin/scripts/js/login.js?v=0:105
    jQuery 6
    doLogin https://<host>/admin/scripts/js/login.js?v=0:91
    <anonymous> https://<host>/admin/scripts/js/login.js?v=0:122
    jQuery 9
    <anonymous> https://<host>/admin/scripts/js/login.js?v=0:111

How does this PR accomplish the above?:

We confirm that responseJSON exists in the response object before accessing into it.

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[bobrippling]

Can't get round the linter - it complains about either eqeqeq or no-eq-null

Removed those changes, left it at responseJSON