Show error code and description only in verbose mode if 'perf probe'
command failed.

Current 'perf probe' shows error code with final error message, and that
is meaningless for many users.

This changes error messages to show the error code and its description
only in verbose mode (-v option).

Without this patch:
  -----
  # perf probe -a do_execve@hoge
  Probe point 'do_execve@hoge' not found.
    Error: Failed to add events. (-2)
  -----

With this patch, normally the message doesn't show the misterious error
number:
  -----
  # perf probe -a do_execve@hoge
  Probe point 'do_execve@hoge' not found.
    Error: Failed to add events.
  -----

And in verbose mode, it also shows additional error messages as below:
  -----
  # perf probe -va do_execve@hoge
  probe-definition(0): do_execve@hoge
  symbol:do_execve file:hoge line:0 offset:0 return:0 lazy:(null)
  0 arguments
  Looking at the vmlinux_path (6 entries long)
  Using /lib/modules/3.15.0-rc8+/build/vmlinux for symbols
  Open Debuginfo file: /lib/modules/3.15.0-rc8+/build/vmlinux
  Try to find probe point from debuginfo.
  Probe point 'do_execve@hoge' not found.
    Error: Failed to add events. Reason: No such file or directory (Code: -2)
  -----
Reported-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20140606071352.6788.76943.stgit@kbuild-fedora.novalocalSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>