Poller errors occur if a file exists that the website cannot read

[netniV]

Describe the bug
If a file exists that the website cannot read, poller errors can occur.

2019/03/26 21:42:02 - CMDPHP PHP ERROR WARNING Backtrace: (/poller.php[187]:update_resource_cache(), /lib/poller.php[611]:cache_in_path(), /lib/poller.php[728]:md5sum_path(), /lib/poller.php[1016]:md5_file(), CactiErrorHandler())
2019/03/26 21:42:02 - ERROR PHP WARNING: md5_file(/usr/share/cacti/site/nano.save.1): failed to open stream: Permission denied in file: /usr/share/cacti/site/lib/poller.php on line: 1016
2019/03/26 21:42:02 - CMDPHP PHP ERROR WARNING Backtrace: (/poller.php[187]:update_resource_cache(), /lib/poller.php[611]:cache_in_path(), /lib/poller.php[728]:md5sum_path(), /lib/poller.php[1016]:md5_file(), CactiErrorHandler())
2019/03/26 21:42:02 - ERROR PHP WARNING: md5_file(/usr/share/cacti/site/nano.save): failed to open stream: Permission denied in file: /usr/share/cacti/site/lib/poller.php on line: 1016

To Reproduce
Steps to reproduce the behavior:

1. Go to website route folder
2. Create a file owned by another user (eg root)
3. See error

Expected behavior
If a file is unreadable due to permissions, a warning should be logged but processing should continue.