[fix] harden get_engine_locale: handle UnknownLocaleError exceptions

When a user selects an unknown or invalid locale by using the search syntax:

    !qw siemens :de-TW

Before this patch a UnknownLocaleError exception will be rasied:

```
Traceback (most recent call last):
  File "SearXNG/searx/search/processors/online.py", line 154, in search
    search_results = self._search_basic(query, params)
  File "SearXNG/searx/search/processors/online.py", line 128, in _search_basic
    self.engine.request(query, params)
  File "SearXNG/searx/engines/qwant.py", line 98, in request
    q_locale = get_engine_locale(params['language'], supported_languages, default='en_US')
  File "SearXNG/searx/locales.py", line 216, in get_engine_locale
    locale = babel.Locale.parse(searxng_locale, sep='-')
  File "SearXNG/local/py3/lib/python3.8/site-packages/babel/core.py", line 330, in parse
    raise UnknownLocaleError(input_id)
```

This patch implements a simple exception handling, since e.g. `de-TW` does not
exists `de` will be used to get engines locale.  On invalid terms like `xy-XY`
the default will be returned.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>