perf ftrace: Detect workload failure

Currently there's no error message prompted if we failed to start
workload.  And we still get some trace which is confusing. Let's tell
users what happened.

Committer testing:

Before:

    # perf ftrace nonsense |& head
     5)               |  switch_mm_irqs_off() {
     5)   0.400 us    |    load_new_mm_cr3();
     5)   3.261 us    |  }
     ------------------------------------------
     5)    <idle>-0    =>   <...>-3494
     ------------------------------------------

     5)               |  finish_task_switch() {
     5)   ==========> |
     5)               |    smp_irq_work_interrupt() {
    # type nonsense
    -bash: type: nonsense: not found
    #

After:

  # perf ftrace nonsense |& head
  workload failed: No such file or directory
  # type nonsense
  -bash: type: nonsense: not found
  #

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200510150628.16610-3-changbin.du@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>