sync: ignore --delete-removed if no source files, bug #155

https://github.com/s3tools/s3cmd/issues/155

If --delete-removed is specified, yet no source files were found, all
files at destination are deleted.  This is more likely a user error
than the desired outcome.

This patch adds a check to [sync] for both --delete-removed and no
source files, and refuses to delete on the destination in this case.
The user can specify --force to force deletion if that is really what
is intended.