perf ui/tui: Show fatal error message only if exists

perf ui/tui: Show fatal error message only if exists

When perf exits with some error it shows the error message with
ui__error() or ui__warning() and then calls ui__exit() during
exit_browser().

On TUI, it then shows a window titled "Fatal Error" to inform user a
last message which might be related with this condition.  However it
sometimes contains no message and just annoyes users.

The usual case for this is running perf top as normal user.  (And
/proc/sys/kernel/perf_event_paranoid being 1).

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1421736050-5283-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>