tracing/fastboot: put error message on stderr

Since this scripts output is usually redirected, put error messages
on standard error and exit with error code if no data is found.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>