set default value and fail hard on miss-config

cmd/flags.go

@@ -109,7 +109,7 @@ var (
 			Name:    "pages-branch",
 			Usage:   "define a branch to fetch assets from",
 			EnvVars: []string{"PAGES_BRANCHES"},
-			Value:   &cli.StringSlice{},
+			Value:   cli.NewStringSlice("pages"),
 		},
 
 		// ############################

cmd/main.go

@@ -63,7 +63,7 @@ func Serve(ctx *cli.Context) error {
 	}
 
 	if len(defaultBranches) == 0 {
-		defaultBranches = []string{"pages"}
+		return fmt.Errorf("no default branches set (PAGES_BRANCHES)")
 	}
 
 	// Init ssl cert database