the override keyword

[Frank Benoit]

Can we make the override keyword also be needed for implementing an
interface function.
This is not exactly overriding, but the purpose is the same. I want to
write the keyword to say "Mr Compiler, check the override/implementing
rules for a definition in the superclass or an interface! If the sig
doens't match one, give error.".

Another feature request here is:
The message "cannot instanciate abstract class XYZ..." is not so
helpful. Please give a tip, of at least one missing implementation. I
had several times the case where I needed to search a hierarchy for long
time to find out why a class is abstract. This is kind of more
complicated with D2 where a const thing changes the signature and
override is also needed.