kbucket: fix exception crash caused by bad removeContact()

fixes crash introduced in 70d08eae9ce23bf259643cc3a175440ec2bc762a
(Raise value error when removing non-existing contact.)

Signed-off-by: Thomas Stilwell <stilwellt@openlabs.co>
Merge pull request #564 from benemorius/crashfix

kbucket: fix exception crash caused by bad removeContact()