refactor constant evaluation error reporting

Refactor constant evaluation to use a single error reporting function
that reports a type-error-like message.

Also, unify all error codes with the "constant evaluation error" message
to just E0080, and similarly for a few other duplicate codes. The old
situation was a total mess, and now that we have *something* we can
further iterate on the UX.

src/test/compile-fail/associated-const-array-len.rs

@@ -14,7 +14,7 @@ trait Foo {
     const ID: usize;
 }
 
-const X: [i32; <i32 as Foo>::ID] = [0, 1, 2]; //~ ERROR E0250
+const X: [i32; <i32 as Foo>::ID] = [0, 1, 2]; //~ ERROR E0080
 
 fn main() {
     assert_eq!(1, X);