Rlp decode returns Result (#8527)

rlp::decode returns Result

Make a best effort to handle decoding errors gracefully throughout the code, using `expect` where the value is guaranteed to be valid (and in other places where it makes sense).

ethcore/src/trace/types/flat.rs

@@ -244,7 +244,7 @@ mod tests {
 		]);
 
 		let encoded = ::rlp::encode(&block_traces);
-		let decoded = ::rlp::decode(&encoded);
+		let decoded = ::rlp::decode(&encoded).expect("error decoding block traces");
 		assert_eq!(block_traces, decoded);
 	}
 }