These annotations are meant to warn from inheriting a class or
from overriding a member, due to the reasons given in `msg`.

The naming and placement of the methods is in line with
@deprecated and @deprecatedName.

While they ought to be generalized to aribirary modifier
changes before being offered in the standard library, the
opportunity to use them in 2.10 is too important to pass up.

So for now, they're private[scala].

En route:

 - made the error messages more concise
 - fix positioning of inheritance error
 - improve test coverage