milo: Add a build info Swarming implementation.

milo/appengine/swarming/html.go

Index: milo/appengine/swarming/html.go
diff --git a/milo/appengine/swarming/html.go b/milo/appengine/swarming/html.go
index 5fb3bea18f00ef2a13ea75e620131be35a50dbe1..2b1b3e76de5843c1ad364e4b21aa85b087e2b8b9 100644
--- a/milo/appengine/swarming/html.go
+++ b/milo/appengine/swarming/html.go
@@ -8,11 +8,12 @@ import (
 	"net/http"
 	"os"
 
-	"google.golang.org/api/googleapi"
-
 	"github.com/julienschmidt/httprouter"
 	"golang.org/x/net/context"
+	"google.golang.org/api/googleapi"
+	"google.golang.org/grpc/codes"
 
+	"github.com/luci/luci-go/grpc/grpcutil"
 	"github.com/luci/luci-go/milo/appengine/settings"
 	"github.com/luci/luci-go/milo/common/miloerror"
 	"github.com/luci/luci-go/server/templates"
@@ -31,6 +32,8 @@ func getSwarmingService(c context.Context, r *http.Request) (swarmingService, er
 		server = defaultSwarmingServer
 	case "dev":
 		server = defaultSwarmingDevServer
+	default:
+		return nil, grpcutil.Errf(codes.InvalidArgument, "invalid swarming server")
 	}
 	return newProdService(c, server)
 }