Raise NameError if model specified in ADMIN_REORDER can't be found

[pief]

Makes it easier to catch configuration mistakes that otherwise just tend to get overseen.

[coveralls]

Coverage decreased (-0.009%) to 0.962% when pulling 792f2a4 on pief:master into 6e3ea39 on mishbahr:master.

[jedie]

IMHO a great idea!

But you can also raise an error, of 'app' is not found, too.

[pief]

Not sure I understand?

[jedie]

Ah, i see. Wrong app name seems to raise an error, already.

[emilyzzz]

" Wrong app name seems to raise an error, already.", I'm not seeing this.
I agree with original author with this PR, adding this PR will definitely help identify problem. I had a typo in my app name and took me a while to figure out why the app/models won't show up on admin list.