perf list: Show error if tracepoints not available

Tracepoints are not visible in "perf list" on Fedora 19 because regular
users have no permission to /sys/kernel/debug by default. Show an error
message so that the user knows about it instead of assuming that
tracepoints are not supported on the system.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1381867647-8594-1-git-send-email-penberg@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>