Add '()' annotation and error handling to submit_header

Author: GideonBature

integration_test/tests/mining.rs

@@ -224,5 +224,5 @@ fn mining__submit_header() {
     // Change the nonce to ensure the header is different from the current tip.
     header.nonce = header.nonce.wrapping_add(1);
 
-    let _ = node.client.submit_header(&header);
+    let _: () = node.client.submit_header(&header).expect("submitheader");
 }