Better handling of "--no-traverse" by "rclone sync"

[DurvalMenezes]

As discussed here, when "--no-traverse" is specified as an option to "rclone sync", the code should be changed to:

1. Raise the warning for "--no-traverse" to be an ErrorLog (like the other messages in that bit of code)
2. Patch the docs to explicitly state sync is not compatible with --no-traverse
3. Remove the remote name from the log message (it isn't relevant to any particular remote) which will make it stand out more

[ncw]

I will do this once #517 has landed.

[ncw]

I've done this in this beta

http://beta.rclone.org/v1.35-77-gc0ad29c/ (uploaded in 15-30 mins)