feed: Raise the new InvalidFeedConfig on missing feed.url

You can't fetch a feed without a URL.  This new error message makes
the cause explicit, compared to the somewhat ambigious former error
messages:

  fetch $NAME (None -> $TO)
  process $NAME (None -> $TO)
  HTTP status 200
  could not get HTTP headers: $NAME (None -> $TO)
  unrecognized version: $NAME (None -> $TO)
  sax parsing error: <unknown>:2:0: no element found: $NAME (None -> $TO)

I was getting URL-less feeds when I clobbered my
~/.config/rss2email/config [1], removing some newer entries.  However,
because I never deleted the feeds explicitly, they were repopulated
(without their URL) from ~/.config/rss2email/feeds.dat, and subsequent
runs generated the above error.

[1]: The clobbering was related to my dotfile management, and not due
to an rss2email issue.

Signed-off-by: W. Trevor King <wking@tremily.us>