tools: iio: print error message when buffer enable fails

Running generic_buffer without enabling any channel of the
sensor will fail without printing any error message.

Add an error message that indicates buffer enable failed.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>