Bluetooth: Mesh: Add error checks for scan start/stop

It may be very important information for the developer if scanning
fails to start or stop, so add proper logs. There's no need to log
EALREADY errors, since the mesh stack doesn't track the current scan
state and simply relies on the under lying GAP layer to return this
error if the desired state is already set.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>