message: Add is_closed method to Error
diff: @@ -421,6 +421,11 @@ impl Error {
  self.source().and_then(|e| e.downcast_ref::<DbError>())
  }
  + /// Determines if the error was associated with closed connection.
  + pub fn is_closed(&self) -> bool {
  + self.0.kind == Kind::Closed
  + }
  +
  /// Returns the SQLSTATE error code associated with the error.
  ///
  /// This is a convenience method that downcasts the cause to a `DbError` and returns its code.
  