cmd/go: use normal code 2 for 'no such tool'

Exit code 3 is unprecedented and inconsistent with other failures here,
such as having no tool directory.

Fixes #17145

Change-Id: Ie7ed56494d4511a600214666ce3a726d63a8fd8e
Reviewed-on: https://go-review.googlesource.com/31253
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/tool.go

@@ -60,7 +60,7 @@ func tool(toolName string) string {
 		} else {
 			fmt.Fprintf(os.Stderr, "go tool: no such tool %q\n", toolName)
 		}
-		setExitStatus(3)
+		setExitStatus(2)
 		exit()
 	}
 	return toolPath