feeds: Raise an exception when adding a feed with a duplicate name

Avoid:

  $ r2e add example http://example.com/feed1
  $ r2e add example http://example.com/feed2
  $ r2e list
  1: [*] example (http://example.com/feed2 -> lkmorlan)
  2: [*] example (http://example.com/feed2 -> lkmorlan)

Instead, the second addition now prints:

  duplicate feed name 'example'

to stderr and exits with status 1.

Reported-by: Liam K Morland <Liam@Morland.ca>
Signed-off-by: W. Trevor King <wking@tremily.us>