perf record: Fix fast task-exit race

Recording with -a (or with -p) can race with tasks going away:

   couldn't open /proc/8440/maps

Causing an early exit() and no recording done.

Do not abort the recording session - instead just skip that task.

Also, only print the warnings under -v.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>