remove panic on invalid fn args

src/args.rs

@@ -226,7 +226,7 @@ pub(crate) fn eat_func_args<I: Iterator<Item = Token>>(
                 close_paren_span = pos;
                 break;
             }
-            _ => todo!(),
+            _ => return Err(("expected \")\".", pos).into()),
         };
         let mut default: Vec<Token> = Vec::new();
         let mut is_variadic = false;

tests/functions.rs

@@ -83,3 +83,11 @@ test!(
     "@function str-replace($string, $search, $replace: \"\") {\n  $index: $string;\n  @return $index;\n}\n\na {\n  color: str-replace(\"a#b#c\", \"#\", \":\");\n}",
     "a {\n  color: \"a#b#c\";\n}\n"
 );
+error!(
+    missing_name,
+    "@function() {}", "Error: Expected identifier."
+);
+error!(
+    args_do_not_start_with_var,
+    "@function foo(FOO) {}", "Error: expected \")\"."
+);