locale doesn't preserve territory and doesn't do error handling

[spaceone]

>>> import tornado.locale
>>> tornado.locale.load_gettext_translations('/usr/share/locale', 'foo')

>>> tornado.locale.get('de').code
'de'
>>> tornado.locale.get('de_DE').code
'de'
>>> tornado.locale.get('de-DE').code
'de'
>>> tornado.locale.get('de-DE').__dict__.keys()
dict_keys(['ngettext', 'gettext', 'code', 'name', 'rtl', 'translations', '_months', '_weekdays'])

→ there is no territory information stored. Please store it additionally.
I provide a de locale directory which is valid for de-DE and de-AT. I want to pass the exact value back which the client provided in the Accept-Language header.

>>> tornado.locale.get('de')
<tornado.locale.GettextLocale object at 0x7f4adae9ca58>
>>> tornado.locale.get('de-DE')
<tornado.locale.GettextLocale object at 0x7f4adae9ca58>
>>> tornado.locale.get('de_DE')
<tornado.locale.GettextLocale object at 0x7f4adae9ca58>
>>> tornado.locale.get('de_DE-foo')
<tornado.locale.CSVLocale object at 0x7f4ade726e80>

→ why doesn't this raise some exception?

see also #1955 and #1858

Edit: there is some assertion error. Maybe this can also be changed into a real exception?

>>> tornado.locale.Locale.get('en_US')
<tornado.locale.CSVLocale object at 0x7f333cc632e8>
>>> tornado.locale.Locale.get('en')                                                                                                                                                                                                           
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/tornado/locale.py", line 264, in get
    assert code in _supported_locales
AssertionError