perf tool: report user-friendly errors from trace

Currently, execution of 'perf trace' reports the following cryptic
message to the user:

$ perf trace
Couldn't read the raw_syscalls tracepoints information!

Typically this happens because the user does not have permissions to
read the debugfs filesystem. Also handle the case when the kernel was
not compiled with debugfs support or when it isn't mounted.

Now, the tool prints detailed error messages:

$ perf trace
Error:  Unable to find debugfs
Hint:   Was your kernel was compiled with debugfs support?
Hint:   Is the debugfs filesystem mounted?

$ perf trace
Error:  No permissions to read $debugfs/tracing/events/raw_syscalls
Hint:   Change the permissions of the debugfs filesystem

Cc: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>