command: Set the 'feed' argument when raising NoToEmailAddress

Before this commit, if no default target email was setup in the
config:

  $ r2e add example http://example.net/
  Traceback (most recent call last):
    File "/usr/bin/r2e", line 5, in <module>
      rss2email.main.run()
    File "/usr/lib/python3/dist-packages/rss2email/main.py", line 163, in run
      args.func(feeds=feeds, args=args)
    File "/usr/lib/python3/dist-packages/rss2email/command.py", line 50, in add
      raise _error.NoToEmailAddress(feeds=feeds)
  TypeError: __init__() missing 1 required positional argument: 'feed'

After this commit:

  $ r2e add example http://example.net/
  no target email address has been defined

I added the kwargs handling to FeedError so it will be passed through
to FeedsError because NoToEmailAddress has a diamond-inheritence that
includes both RSS2EmailError subclasses.

Reported-by: Jakub Wilk <jwilk@debian.org>
Signed-off-by: W. Trevor King <wking@tremily.us>
CHANGELOG: Document this branch

Documents the following commits:
* 763d9a8 command: Set the 'feed' argument when raising NoToEmailAddress

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