message: Fix type in CHANGELOG: ParseError -> StdError::ParseErr
diff: - Rename `Context::add_log` to `Context::add_attribute`.
  - Add `Api::debug` for emitting debug messages during development.
  - Fix error type for response parsing errors in `ExternalQuerier::raw_query`.
  - This was `Ok(Err(ParseError))` instead of `Err(SystemError::InvalidResponse)`,
  - implying an error created in the target contract.
  + This was `Ok(Err(StdError::ParseErr))` instead of
  + `Err(SystemError::InvalidResponse)`, implying an error created in the target
  + contract.
  - Deprecate `StdError::Unauthorized` and `StdError::unauthorized` in favour of
  custom errors. From now on `StdError` should only be created by the standard
  library and should only contain cases the standard library needs.
  