sync: better warning when syncing a detached head
sync: better warning when syncing a detached head

note that I'm not sure which is better:
"is on a detached HEAD" or
"is not on any branch"

Change-Id: I009e515da4dbcd09be48766c581fa8225ef4b925
Reviewed-on: http://gerrit.aldebaran.lan:8080/3118
Reviewed-by: dmerejkowsky <1e4b13000fa8777df4cefd2fd8215187e3deb062@aldebaran-robotics.com>
Tested-by: sbarthelemy <9c154f64c36738f8e44d945c231ac417d15bd66e@aldebaran-robotics.com>

aldebaran/qibuild,aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild