1150562: Fixed exception in CP when re-registering a pool/product to activation keys

- CP will no longer throw an exception when a client attempts to add a pool
  or product to an activation key multiple times. Instead, a proper error
  message is returned to the client.
Merge pull request #782 from candlepin/crog/1150562

1150562: Fixed exception in CP when re-registering a pool/product to keys