perf tools: Convert critical messages to ui__error()

There were places where use ui__warning (or even fprintf) to show
critical messages. This patch converts them to ui__error so that the
front-end code can implement appropriate behavior.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1338265382-6872-3-git-send-email-namhyung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>