Make verbose rpc error display (#758)

* Make verbose rpc erro display

* Apply review results

* Minor fix doc-comment

subxt/src/error.rs

@@ -107,13 +107,14 @@ impl From<DispatchError> for Error {
 /// An RPC error. Since we are generic over the RPC client that is used,
 /// the error is boxed and could be casted.
 #[derive(Debug, thiserror::Error)]
-#[error("RPC error")]
 pub enum RpcError {
     // Dev note: We need the error to be safely sent between threads
     // for `subscribe_to_block_headers_filling_in_gaps` and friends.
     /// Error related to the RPC client.
+    #[error("RPC error: {0}")]
     ClientError(Box<dyn std::error::Error + Send + Sync + 'static>),
     /// The RPC subscription dropped.
+    #[error("RPC error: subscription dropped.")]
     SubscriptionDropped,
 }