go/types: error reporting for misplaced fallthrough could be better

[ALTree]

On tip, on the following program:

package p
func g() {
    var x int
    switch x {
    case 1:
        fallthrough
    }
}

go/types reports a generic

test.go:7:3: fallthrough statement out of place

gc's error is more specific:

./test.go:7: cannot fallthrough final case in switch

[griesemer]

This is not a regression, it has always been like this for go/types. But agreed that the error message could be more specific.