go/types: provide better error message for errors in n:1 init assignment

[griesemer]

Given a.go:

package p

var a, b []int = f1() // error

func f1() (_ []int, _ int) {
    return
}

gotype reports:

$ gotype a.go
a.go:3:18: cannot use f1() (value of type int) as []int value in assignment

vs gc compiler:

$ go tool compile a.go
a.go:3: cannot assign int to b (type []int) in multiple assignment

The gc compiler's error message is much better.

[griesemer]

Not urgent. Pushing to 1.9 Maybe.

[griesemer]

Still not urgent. Pushing to 1.12.