Changed the `files` parameter to be mandatory and added check to
ensure that at least one file is passed in configuration otherwise
the instrument crashes.
- Updated poller binary to propery check for errors (e.g. on attempting
to open a file) and report them to stderr
- Updated the file_poller instrument to collect poller stderr output
into a log file and to check the log for errors or warnings on
completion of an iteration.
- device.is_rooted check was being perfromed inside validate() which
gets invoked before the device is connected. This worked for most
Android devices, because connections is a no-op for them, however
failed for Linux targets. The check is now performed inside
initialize().
- Added _is_ready() check inside is_rooted to catch similar problems
quicker in the future.
