improve error messages for sealed classes

rather than:

  class is not allowed to extend sealed class: {0}

consider:

  class {0} in unnamed module cannot extend a sealed class in a different package

  class {0} in module {1} cannot extend a sealed class in a different module

reported in amber-dev: https://mail.openjdk.java.net/pipermail/amber-dev/2020-June/006185.html

also:
Compiling this program

public class Main {
    public static void main(String[] args) {
        Shape shape = new Shape() {};
    }
}

sealed interface Shape {
}

leads to an error:

error: local classes must not extend sealed classes
        Shape shape = new Shape() {};
                                  ^

But the class of `shape` is an anonymous class rather than a local class.
By definition, a local class must have a name (JLS 14.3). I think the error
message should say " anonymous classes must not extend sealed classes".

reported at amber-dev: https://mail.openjdk.java.net/pipermail/amber-dev/2020-June/006186.html