resolve panic on malformed args missing closing parenthesis

src/parse/args.rs

@@ -345,9 +345,9 @@ impl<'a> Parser<'a> {
                         None => return Err(("expected \")\".", span).into()),
                     }
                 }
-                Some(c) => {
+                Some(Token { pos, .. }) => {
                     value?;
-                    unreachable!("{:?}", c)
+                    return Err(("expected \")\".", *pos).into());
                 }
                 None => return Err(("expected \")\".", span).into()),
             }

tests/args.rs

@@ -168,6 +168,10 @@ error!(
     filter_value_after_equal_is_last_char,
     "a {\n  color: foo(a=a", "Error: expected \")\"."
 );
+error!(
+    unclosed_paren_in_nested_args,
+    "a { color: a(b(red); }", "Error: expected \")\"."
+);
 test!(
     space_after_loud_comment,
     "@mixin foo($x) {