Fix lack of message when setting exec bit

Nothing was telling that the content couldn't be
set as executable. Note that nothing is made to check
that a content (executable or not) exist.
Also note that installing several executables for
one widget isn't possible now.

Change-Id: I10373fa27cd8d1d90c292e3529d8a5eec62bb8f0
Signed-off-by: José Bollo <jose.bollo@iot.bzh>