RFR 8057650: uniform error diagnostics for inconsistent inherited method signatures

[Maurizio Cimadamore]

Hi,
the following is a simple patch which consolidates similar diagnostics 
caused by incompatible overrides - the diagnostics are all of the form:

type A and B are incompatible because...

So I created a toplevel key for the 'type are incompatible' bit, and a 
bunch of fragments for the various possible erroneous situations. See 
webrev here:

http://cr.openjdk.java.net/~mcimadamore/8057650/

As usual, I've also attached some examples of the touched diagnostic 
keys, as a reference.

Note that, since I was there, I made one of the diagnostics better - the 
key for 'diff.ret' used to take a String argument, while now takes a 
(method) name and a list of types (the parameters), which is consistent 
with other similar diagnostics, and avoids premature string-ification.

Cheers
Maurizio