verification: fix race same block + misc (#11400)

* ethcore: fix race in verification

* verification: fix some nits

* verification: refactor err type `Kind::create`

* fix: tests

* address grumbles

* address grumbles: don't panic

ethcore/light/src/client/mod.rs

@@ -220,7 +220,7 @@ impl<T: ChainDataFetcher> Client<T> {
 
 	/// Import a header to the queue for additional verification.
 	pub fn import_header(&self, header: Header) -> EthcoreResult<H256> {
-		self.queue.import(header).map_err(|(_, e)| e)
+		self.queue.import(header).map_err(|(e, _)| e)
 	}
 
 	/// Inquire about the status of a given header.