Proper error messages for too many arguments

src/builtin/color/other.rs

@@ -174,6 +174,7 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         }))
     });
     decl!(f "ie-hex-str", |args, _| {
+        max_args!(args, 1);
         let color = match arg!(args, 0, "color").eval() {
             Value::Color(c) => c.clone(),
             v => return Err(format!("$color: {} is not a color.", v).into()),

src/builtin/color/rgb.rs

@@ -175,12 +175,12 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
     decl!(f "mix", |args, _| {
         let color1 = match arg!(args, 0, "color1").eval() {
             Value::Color(c) => c,
-            v => return Err(format!("$color: {} is not a color.", v).into()),
+            v => return Err(format!("$color1: {} is not a color.", v).into()),
         };
 
         let color2 = match arg!(args, 1, "color2").eval() {
             Value::Color(c) => c,
-            v => return Err(format!("$color: {} is not a color.", v).into()),
+            v => return Err(format!("$color2: {} is not a color.", v).into()),
         };
 
         let weight = match arg!(args, 2, "weight"=Value::Dimension(Number::from(50), Unit::None)) {

src/builtin/macros.rs

@@ -24,3 +24,11 @@ macro_rules! decl {
         $f.insert($name.to_owned(), Box::new($body));
     };
 }
+
+macro_rules! max_args {
+    ($args:ident, $count:literal) => {
+        if $args.len() > $count {
+            return Err(format!("Only {} argument allowed, but {} were passed.", $count, $args.len()).into());
+        }
+    };
+}