Touch-up error-message and godoc for ConfigVXLANUDPPort
Touch-up error-message and godoc for ConfigVXLANUDPPort

Minor changes following review of the engine pull request
for this feature;

- Remove the name of the function from the error message
  as it's not a debug message.
- Add the valid range to the error message, so that a
  user has sufficient information to address the problem.
- Update GoDoc for the function to describe the default
  port, and valid port-ranges.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/libnetwork,mahak/libnetwork,medallia/libnetwork,mahak/libnetwork,docker/libnetwork,mahak/libnetwork,medallia/libnetwork,docker/libnetwork,medallia/libnetwork