message: Add back Error::as_db_error
  Closes
diff: @@ -414,14 +414,18 @@ impl Error {
  self.0.cause
  }
  + /// Returns the source of this error if it was a `DbError`.
  + ///
  + /// This is a simple convenience method.
  + pub fn as_db_error(&self) -> Option<&DbError> {
  + self.source().and_then(|e| e.downcast_ref::<DbError>())
  + }
  +
  /// 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.
  + /// This is a convenience method that downcasts the cause to a `DbError` and returns its code.
  pub fn code(&self) -> Option<&SqlState> {
  - self.source()
  - .and_then(|e| e.downcast_ref::<DbError>())
  - .map(DbError::code)
  + self.as_db_error().map(DbError::code)
  }
  fn new(kind: Kind, cause: Option<Box<dyn error::Error + Sync + Send>>) -> Error {
  