Make StorageError initializer public

[AngCosmin]

I made the initializer from StorageError public so it can be used from outside of the package. This allows developers to make wrappers over certain functions and they can return StorageError as result if needed.

For example, I've created this extension over StorageFileApi and I would like to return StorageError if I cannot convert UIImage to Data

[grdsdev]

Thanks @AngCosmin