Expand discriminant overflow error message

Renders as:

    error[cxxbridge]: discriminant overflow on value after 4294967295
       ┌─ src/main.rs:11:9
       │
    11 │         B,
       │         ^ discriminant overflow
       │
       = note: explicitly set `= 0` if that is desired outcome

This more closely matches rustc's error message, which is:

    error[E0370]: enum discriminant overflowed
     --> src/lib.rs:4:5
      |
    4 |     B,
      |     ^ overflowed on value after 255
      |
      = note: explicitly set `B = 0` if that is desired outcome
diff --git a/syntax/parse.rs b/syntax/parse.rs
index 1f6cbac..7e4422d 100644
--- a/syntax/parse.rs
+++ b/syntax/parse.rs
@@ -112,7 +112,8 @@
             }
         }
         if variant.discriminant.is_none() && prev_discriminant.unwrap_or(0) == u32::MAX {
-            return Err(Error::new_spanned(variant, "overflowed on value"));
+            let msg = format!("discriminant overflow on value after {}", u32::MAX);
+            return Err(Error::new_spanned(variant, msg));
         }
         let discriminant =
             parse_discriminant(&variant)?.unwrap_or_else(|| prev_discriminant.map_or(0, |n| n + 1));