perf tool: report user-friendly error from timechart

Earlier, when 'timechart record' was invoked by a user without
permissions to debugfs:

$ perf timechart record
invalid or unsupported event: 'power:cpu_frequency'
Run 'perf list' for a list of valid events

 usage: perf record [<options>] [<command>]
    or: perf record [<options>] -- <command> [<options>]
[... detailed usage information ...]

This is highly user unfriendly, as the real problem is not reported to
the user correctly. Now, the user gets a much more friendly:

$ perf timechart record
Error:  No permissions to read $debugfs/tracing/events/power/cpu_frequency
Hint:   Change the permissions of debugfs: /sys/kernel/debug
        The directory will be present if your kernel was compiled with debugfs support.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>