Update error message to clarify that it's not the enum itself that's non_exhaustive

Author: scottmcm

compiler/rustc_typeck/src/check/cast.rs

@@ -601,7 +601,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     self.cast_ty,
                     fcx,
                 )
-                .note("cannot cast a non-exhaustive enum defined in another crate")
+                .note("cannot cast an enum with a non-exhaustive variant when it's defined in another crate")
                 .emit();
             }
         }

src/test/ui/rfc-2008-non-exhaustive/enum-as-cast.stderr

@@ -4,7 +4,7 @@ error[E0606]: casting `FieldLessWithNonExhaustiveVariant` as `u8` is invalid
 LL |     let d = e as u8;
    |             ^^^^^^^
    |
-   = note: cannot cast a non-exhaustive enum defined in another crate
+   = note: cannot cast an enum with a non-exhaustive variant when it's defined in another crate
 
 error: aborting due to previous error