perf test: Clarify some error reports in the open syscall test

Rebooted my devel machine, first thing I ran was perf test, that expects
debugfs to be mounted, test fails. Be more clear about it.

Also add missing newlines and add more informative message when
sys_perf_event_open fails.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>