rpn(1): changes to error message

src/rpn.rs

@@ -129,18 +129,18 @@ fn eval(
 		};
 
 		match x {
-			Val(x_) => stack.push_back(x_),
+			Val(v) => stack.push_back(v),
 			_ => ops.push_back(x),
 		}
 	}
 
 	for op in &ops {
 		match op {
-			Val(_) => {
-				return Err(EvaluationError {
-					message: "Unexpected value in the operator stack."
-						.to_string()
-				})
+			Val(v) => {
+				return Err(EvaluationError { message: format!(
+					"{}: Unexpected value in the operator stack.",
+					v
+				)});
 			},
 			_ => {
 				let (x, y) = (