perf stat: When no events, don't report an error if there is none

perf stat: When no events, don't report an error if there is none

Events may fail to open as no supported CPUs were specified on the
command line. In this case a confusing "error" message of "success"
can be reported. Let's skip the error in that case.

Before:
```
$ perf stat -C2048 -e cycles -- true
WARNING: A requested CPU in '2048' is not supported by PMU 'cpu' (CPUs 0-7) for event 'cycles'
Error:
No supported events found.
The sys_perf_event_open() syscall returned with 0 (Success) for event (cpu/unknown-hardware/).
"dmesg | grep -i perf" may provide additional information.
```

After:
```
$ perf stat -C2048 -e cycles -- true
WARNING: A requested CPU in '2048' is not supported by PMU 'cpu' (CPUs 0-7) for event 'cycles'
Error:
No supported events found.
```

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>