fix: silence usage, and no double errors

cmd/lambda/lambda.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"fmt"
 	"os"
 
 	"github.com/spf13/cobra"
@@ -89,7 +88,6 @@ func Lambda() *cobra.Command {
 func main() {
 	lambda := Lambda()
 	if err := lambda.Execute(); err != nil {
-		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
 }

cmd/lambda/lambda_convert.go

@@ -28,6 +28,7 @@ func LambdaConvert() *cobra.Command {
 		Use:          "convert <input> <output>",
 		Short:        "Convert between lambda calculus representations",
 		Args:         cobra.ExactArgs(2),
+		SilenceUsage: true,
 		RunE: func(_ *cobra.Command, args []string) error {
 			var err error
 			inputPath, outputPath := args[0], args[1]