Exception instead of "Unknown language"

[projetmbc]

The following behavior looks strange to me. I think that it would be better to raise a LanguageTagError in the last case.

>>> from langcodes import get as get_langcode
>>> onelang = get_langcode('XXXXXX').maximize()
Traceback (most recent call last):
...
langcodes.tag_parser.LanguageTagError: Expected a language code, got 'xxxxxx'
>>> onelang = get_langcode('XX').maximize()
>>> describe = onelang.describe('en')
>>> describe
{'language': 'Unknown language [xx]', 'script': 'Latin', 'territory': 'United States'}
~~