Identify "failed to read status" as ErrorKind::Connection errors

So that they will be handled by the retry_request_batch() automatic
retry mechanism, which only applies to connection-related errors.

src/daemon.rs

@@ -208,7 +208,7 @@ impl Connection {
             .chain_err(|| {
                 ErrorKind::Connection("disconnected from daemon while receiving".to_owned())
             })?
-            .chain_err(|| "failed to read status")?;
+            .chain_err(|| ErrorKind::Connection("failed to read status".to_owned()))?;
         let mut headers = HashMap::new();
         for line in iter {
             let line = line.chain_err(|| ErrorKind::Connection("failed to read".to_owned()))?;