feat: better convert usage

cmd/lambda/lambda_convert.go

@@ -25,11 +25,14 @@ func LambdaConvert() *cobra.Command {
 	var inputReprFlag, outputReprFlag string
 
 	cmd := &cobra.Command{
-		Use:          "convert <input> <output>",
+		Use:          "convert <input-file> <output-file>",
 		Short:        "Convert between lambda calculus representations",
-		Args:         cobra.ExactArgs(2),
 		SilenceUsage: true,
-		RunE: func(_ *cobra.Command, args []string) error {
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if len(args) != 2 {
+				return cmd.Help()
+			}
+
 			var err error
 			inputPath, outputPath := args[0], args[1]
 
@@ -71,7 +74,7 @@ func LambdaConvert() *cobra.Command {
 			// Marshal output.
 			output, err := r.Marshal(result)
 			if err != nil {
-				return fmt.Errorf("marshaling output: %w", err)
+				return fmt.Errorf("unmarshaling output: %w", err)
 			}
 
 			// Write output file.