Simplify error handling of FormatProvider.get_format_for_id and get_format_for_name.

With tuple (None, None) returned, following code can be used for error
handling:

(device, fmt) = fmtprovider.get_format_for_name(...)

if not device:
    raise ...
 
With None, the unpacking would not be possible.

Also extend the documentation to explicitly state which device is returned.