Improve the error message of initialization checker by showing the uninitialized fields

[Ao-senXiong]

The issue is from #621.

When the initialization checker issues errors related to uninitialized fields, it would be helpful if it could also indicate which specific fields have not been initialized.

[Ao-senXiong]

@wmdietl I think right now we do show the uninitialized fields in the error message. See the following example:

class Demo {
    String s;
    Object foo;

    Demo() {
        init();
    }

    void init() {}
}

Command : java -jar checker/dist/checker.jar -processor nullness FinalClass.java
Error message:

FinalClass.java:5: error: [initialization.fields.uninitialized] the constructor does not initialize fields: s, foo
    Demo() {
    ^
FinalClass.java:6: error: [method.invocation.invalid] call to init() not allowed on the given receiver.
        init();
            ^
  found   : @UnderInitialization Demo
  required: @Initialized Demo
2 errors

[wmdietl]

Try the following changed constructor:

    Demo() {
        s = "";
        init();
        foo = "";
    }

[Ao-senXiong]

Great! Thanks!