cmd/compile: improve assignment count mismatch error message

Given the following test cases:

    $ cat left_too_many.go
    package main

    func main() {
    	a, err := make([]int, 1)
    }

    $ cat right_too_many.go
    package main

    func main() {
    	a := "foo", "bar"
    }

Before this change, the error messages are:

    ./left_too_many.go:4: assignment count mismatch: 2 = 1

    ./right_too_many.go:4: assignment count mismatch: 1 = 2

After this change, the error messages are:

    ./left_too_many.go:4: assignment count mismatch: want 2 values, got 1

    ./right_too_many.go:4: assignment count mismatch: want 1 values, got 2

Change-Id: I9ad346f122406bc9a785bf690ed7b3de76a422da
Reviewed-on: https://go-review.googlesource.com/33616Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/typecheck.go

@@ -3429,7 +3429,7 @@ func typecheckas2(n *Node) {
 	}
 
 mismatch:
-	yyerror("assignment count mismatch: %d = %d", cl, cr)
+	yyerror("assignment count mismatch: want %d values, got %d", cl, cr)
 
 	// second half of dance
 out: