simpleperf: improve error msg for output file.

When writing to a read-only file system, the error msg is improved as
below:
$ simpleperf record sleep 1
Can't create output file in directory .: Read-only file system

Bug: 187126442
Test: run simpleperf_unit_test
Change-Id: I5c61514aeba1bfc73e9bf95f625adf76e7056fa0
6 files changed