Improve type.incompatible error messages for wildcards

Compiling the following code:

import java.util.*;

import org.checkerframework.checker.nullness.qual.*;

public class Issue520 {
}

abstract class Parent<T> {
    protected final List<? super @KeyForBottom T> list;

    public Parent(List<? super @KeyForBottom T> list) {
        this.list = list;
    }
}

abstract class Child extends Parent<CharSequence> {
    public Child(List<? super CharSequence> list) {
        super(list);
    }

    public void add(CharSequence seq) {
        list.add(seq);
    }
}

with the Nullness Checker issues the following error:

Issue520.java:26: error: [argument.type.incompatible] incompatible types in argument.
        list.add(seq);
             ^
  found   : @UnknownKeyFor CharSequence
  required: ? extends @UnknownKeyFor Object super @KeyForBottom CharSequence
1 error

That found type is within that required range, so no error should be issued.

A test case appears as file
checker-framework/checker/tests/nullness/Issue520.java

Start with checker/tests/nullness/Issue520.java and run the Nullness Checker test for it. Trace the [argument.type.incompatible] diagnostic produced for list.add(seq), then make the wildcard case pass without reporting an error and verify the test succeeds.