Improve error message about necessary type annotations when generic class member calls the same member on an instance with a concrete class

Consider the following code:

```f#
type Foo<'a>(x: Foo) =

member __.Do() =
x.Do()
()
```

It doesn't compile; it produces one error (on `'a`, "the explicit type variable could not be generalized") and two warnings (on `this` and `x.Do()`, "causes code to be less generic than indicated by the type annotations").

For some reason (I'd appreciate an explanation why), adding the return type annotation `: unit` to `Do()` makes the error and warnings go away.

I suggest to improve the error message in cases like these; it was only by chance that I came across the correct fix.

(Originally reported in https://github.com/fsharp/fsharp/issues/869)