Update two E308 tests to the new placeholder error

src/test/ui/associated-types/higher-ranked-projection.rs

@@ -23,5 +23,5 @@ fn foo<U, T>(_t: T)
 #[rustc_error]
 fn main() { //[good]~ ERROR compilation successful
     foo(());
-    //[bad]~^ ERROR E0308
+    //[bad]~^ ERROR not general enough
 }

src/test/ui/hrtb/hrtb-perfect-forwarding.rs

@@ -43,7 +43,7 @@ fn foo_hrtb_bar_not<'b,T>(mut t: T)
     // be implemented. Thus to satisfy `&mut T : for<'a> Foo<&'a
     // isize>`, we require `T : for<'a> Bar<&'a isize>`, but the where
     // clause only specifies `T : Bar<&'b isize>`.
-    foo_hrtb_bar_not(&mut t); //~ ERROR E0308
+    foo_hrtb_bar_not(&mut t); //~ ERROR not general enough
 }
 
 fn foo_hrtb_bar_hrtb<T>(mut t: T)