Replace placeholder error message

schemars_derive/src/schema_exprs.rs

@@ -76,7 +76,12 @@ pub fn expr_for_repr(cont: &Container) -> Result<SchemaExpr, syn::Error> {
 
     let variants = match &cont.data {
         Data::Enum(variants) => variants,
-        _ => return Err(syn::Error::new(Span::call_site(), "oh no!")),
+        _ => {
+            return Err(syn::Error::new(
+                Span::call_site(),
+                "JsonSchema_repr can only be used on enums",
+            ))
+        }
     };
 
     if let Some(non_unit_error) = variants.iter().find_map(|v| match v.style {