[XCM] Treat recursion limit error as transient in the MQ (#4202)

Changes:
- Add new error variant `ProcessMessageError::StackLimitReached` and
treat XCM error `ExceedsStackLimit` as such.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>

substrate/frame/support/src/traits/messages.rs

@@ -46,6 +46,8 @@ pub enum ProcessMessageError {
 	/// the case that a queue is re-serviced within the same block after *yielding*. A queue is
 	/// not required to *yield* again when it is being re-serviced withing the same block.
 	Yield,
+	/// The message could not be processed for reaching the stack depth limit.
+	StackLimitReached,
 }
 
 /// Can process messages from a specific origin.
@@ -96,6 +98,8 @@ pub trait ServiceQueues {
 	/// - `weight_limit`: The maximum amount of dynamic weight that this call can use.
 	///
 	/// Returns the dynamic weight used by this call; is never greater than `weight_limit`.
+	/// Should only be called in top-level runtime entry points like `on_initialize` or `on_idle`.
+	/// Otherwise, stack depth limit errors may be miss-handled.
 	fn service_queues(weight_limit: Weight) -> Weight;
 
 	/// Executes a message that could not be executed by [`Self::service_queues()`] because it was