Media uploads not meeting the current constraints should be rejected

[boutell]

If constraints are in place, reject uploads that don't meet them with a clear and compassionate message. Later we'll offer gravity or even real cropping.

The current behavior (we accept it, but you don't see it; there's a message at the top of the page about constraints in general, but it was there before you clicked 'add") is too confusing.

We've seen this problem with client projects.

[tboutell]

This can still happen but we have to take cropping into account. With cropping, the only images that would be rejected would be those that are too small to be cropped.

For non-image types we should take all of the constraints into account.