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

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.
