perf tools: Use ui__error() for reporting --fields errors

perf tools: Use ui__error() for reporting --fields errors

So that we can get it working for TUI, where using just pr_err() would
end up making the message emitted to stderr to be erased by the TUI exit
routine restoring the terminal to its previous state.

Now we can see that trying to use a tracepoint field as one of the
--field entries isn't working:

  # perf top --stdio --no-children -e syscalls:sys_enter_write --fields pid,sym,count
  Error:
  Unknown --fields key: `count'
   Usage: perf top [<options>]

        --fields <key[,keys...]>
                          output field(s): overhead, period, sample plus all of sort keys
  #

Cc: Adrian Hunter <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Wang Nan <[email protected]>
Link: https://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>