This makes the caller's life easier. They can catch
mega.errors.EFAILED instead of catching mega.errors.RequestError
and checking the message attribute.
Because these classes inherit from RequestError, and they have a code
and message attribute, anyone currently catching RequestError should
not have any backwards compatibility issues.
Furthermore, this fixes an existing issue in the codebase where
RequestError is raised with a custom string message, which was causing
IndexError since that message wasn't in CODE_TO_DESCRIPTIONS.
