feed: Convert missing/extra key errors to InvalidFeedConfig

feed: Convert missing/extra key errors to InvalidFeedConfig

This way we get the message and not a full traceback, to avoid scaring
users who aren't familiar with Python tracebacks.  Theres not much
information to go on in the new message, but if you crank up the
verbosity, you get:

  $ PYTHONPATH=. ./r2e -c conf -d data -VVV list
  load feed configuration from ['conf']
  loaded configuration from ['conf']
  load feed data from data
  extra configuration key: use_8bit

which seems good enough for me.

Reported-by: Dmitry Bogatov <KAction@gnu.org>
Signed-off-by: W. Trevor King <wking@tremily.us>