Sync: Fix issue of not freeing a block announcement slot (#8006)

* Sync: Fix issue of not freeing a block announcement slot

There was a bug that when the block announcement validation returned an
error, the slot reserved for this validation wasn't freed. This could
lead to a situation where we rejected any block announcement from such a
peer for that the block announcement returned an error multiple times.

* Better logging

* Fuck I'm dumb

* 🤦

substrate/primitives/consensus/common/src/block_validation.rs

@@ -59,6 +59,10 @@ pub trait BlockAnnounceValidator<B: Block> {
 	///
 	/// Returning [`Validation::Failure`] will lead to a decrease of the
 	/// peers reputation as it sent us invalid data.
+	///
+	/// The returned future should only resolve to an error iff there was an internal error validating
+	/// the block announcement. If the block announcement itself is invalid, this should *always*
+	/// return [`Validation::Failure`].
 	fn validate(
 		&mut self,
 		header: &B::Header,