Improve message from inherited implicitNotFound

[som-snytt]

When using the annotation from a supertype, include the name of the required type in the error message.

Also,

scala> @annotation.implicitNotFound("Need one for ${A}")
     | trait T[A] ; trait U extends T[Int] ; implicitly[U] ; implicitly[T[String]]
       trait T[A] ; trait U extends T[Int] ; implicitly[U] ; implicitly[T[String]]
                                                       ^
On line 2: error: Need one for
       trait T[A] ; trait U extends T[Int] ; implicitly[U] ; implicitly[T[String]]
                                                                       ^
On line 2: error: Need one for String

[som-snytt]

Noticed in fthomas/refined#661

[som-snytt]

Lukas's good suggestion to refcheck if an annotation is missing was a bit too much work, but in future it could be enabled under Xlint:implicitNotFound.

Maybe the message change here can also be tweaked. It would be nice if the intersobralator accepted ${this} for the required type. It's not obvious that type args are helpful unless they match, so there are reasons not to include the annotation. I don't remember the original use case.

In future, warn if omitting the custom message because it intersobralates type args that don't match.