perf report/top TUI: Replace pr_err() with ui__error()

pr_err() in TUI mode does not print anyting on the screen and just
quits.

Replace such pr_err() with ui__error().

Before:

  $ perf report -s +
  $

After:

  $ perf report -s +

    ┌─Error:────────────────┐
    │Invalid --sort key: `+'│
    │                       │
    │Press any key...       │
    └───────────────────────┘

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20191114132213.5419-2-ravi.bangoria@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>