Raise a more relevant error when dockerd is not available on the platform.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

daemon_none.go

@@ -1,11 +1,16 @@
+// +build !daemon
+
 package main
 
 import (
 	"fmt"
+	"runtime"
+	"strings"
 )
 
 // CmdDaemon reports on an error on windows, because there is no exec
 func (p DaemonProxy) CmdDaemon(args ...string) error {
 	return fmt.Errorf(
-		"`docker daemon` does not exist on windows. Please run `dockerd` directly")
+		"`docker daemon` is not supported on %s. Please run `dockerd` directly",
+		strings.Title(runtime.GOOS))
 }

daemon_none_test.go

@@ -1,3 +1,5 @@
+// +build !daemon
+
 package main
 
 import (
@@ -9,7 +11,7 @@ func TestCmdDaemon(t *testing.T) {
 	proxy := NewDaemonProxy()
 	err := proxy.CmdDaemon("--help")
 	if err == nil {
-		t.Fatal("Expected CmdDaemon to fail in Windows.")
+		t.Fatal("Expected CmdDaemon to fail on Windows.")
 	}
 
 	if !strings.Contains(err.Error(), "Please run `dockerd`") {

daemon_unix.go

@@ -1,4 +1,4 @@
-// +build !windows
+// +build daemon
 
 package main