Print cause when println!("{}")

> Print like following:
unable to write to a control group file caused by: Os { code: 22, kind: InvalidInput, message: "Invalid argument" }) }

Signed-off-by: Tim Zhang <tim@hyper.sh>

src/error.rs

@@ -66,7 +66,11 @@ impl fmt::Display for Error {
             ErrorKind::Other => "an unknown error".to_string(),
         };
 
-        write!(f, "{}", msg)
+        if let Some(cause) = &self.cause {
+            write!(f, "{} caused by: {:?}", msg, cause)
+        } else {
+            write!(f, "{}", msg)
+        }
     }
 }