Improve error message on where clause on pallet error (#7821)

* improve error message on where clause on pallet error

* Revert "improve error message on where clause on pallet error"

This reverts commit 5a3cc38976813fccef3357833553ce30f5b988ea.

* Revert "Revert "improve error message on where clause on pallet error""

This reverts commit e3b3fca6bc4fa89816f80dbcb82dc4536a9b2549.

substrate/frame/support/test/tests/pallet_ui/error_where_clause.rs

@@ -0,0 +1,23 @@
+#[frame_support::pallet]
+mod pallet {
+	use frame_support::pallet_prelude::Hooks;
+	use frame_system::pallet_prelude::BlockNumberFor;
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::hooks]
+	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {}
+
+	#[pallet::error]
+	pub enum Error<T> where u32: From<u8> {}
+}
+
+fn main() {
+}

substrate/frame/support/test/tests/pallet_ui/error_where_clause.stderr

@@ -0,0 +1,5 @@
+error: Invalid pallet::error, where clause is not allowed on pallet error item
+  --> $DIR/error_where_clause.rs:19:20
+   |
+19 |     pub enum Error<T> where u32: From<u8> {}
+   |                       ^^^^^