Improve error message for non-exhaustive patterns

Changes error message from displaying first found missing constructor witness to showing up to 10, if necessary.

Fixes issue #16884.

src/test/compile-fail/non-exhaustive-pattern-witness.rs

@@ -34,7 +34,7 @@ fn struct_with_a_nested_enum_and_vector() {
 
 fn enum_with_multiple_missing_variants() {
     match Color::Red {
-    //~^ ERROR non-exhaustive patterns: `Red` not covered
+    //~^ ERROR non-exhaustive patterns: `Red`, `Green` not covered
         Color::CustomRGBA { .. } => ()
     }
 }