Check that volume and price fields are filled in

[aspyct]

Currently, we can save a refill without filling in the volume and price.

This should be disallowed. For now, we can simply highlight the fields in red.

[aspyct]

See also issue #3

[aspyct]

We can also highlight the *required in red.

[aspyct]

See also https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/Articles/cdValidation.html