Use sensible default for config.mylang

If the user has not altered user-config.mylang, it is set to
'language' and pywikibot.Site() raises a confusing exception:
UnknownSite: Language language does not exist in family wikipedia

Detect this situation, report a warning, and set the default to
family 'test' and mylang 'test, which is test.wikipedia.org.

Change-Id: I3b890039381c7f8d06dfe6c0d83140ec5f2f3ed6
Merge "Use sensible default for config.mylang"