perf data: Show error message when conversion failed

Show message when errors occurred during conversion setup and conversion
process.

Before this patch:
  $ ./perf data convert --to-ctf=ctf
  $ echo $?
  255

After this patch:
  $ ./perf data convert --to-ctf=ctf
  Error during conversion setup.

Signed-off-by: He Kuang <hekuang@huawei.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jeremie Galarneau <jgalar@efficios.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-xvhr1vf7zav9kkeo9w1hv4uk@git.kernel.org
Link: http://lkml.kernel.org/r/1429372220-6406-2-git-send-email-jolsa@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>