Catch missing expected user/group names in FileCheck

[rcritten]

Related BZ https://bugzilla.redhat.com/show_bug.cgi?id=2218912

If a group or user does not exist then a FileCheck will fail.

{
"source": "ipahealthcheck.ipa.files",
"check": "IPAFileCheck",
"result": "CRITICAL",
"uuid": "2a079793-b6d4-44d5-8bcb-d2d9e1c6e88c",
"when": "20230626110412Z",
"duration": "0.054574",
"kw": {
"exception": ""getpwnam(): name not found: 'systemd-resolve'"",
"traceback": "Traceback (most recent call last):\n File "/usr/lib/python3.9/site-packages/ipahealthcheck/core/core.py", line 56, in run_plugin\n for result in plugin.check():\n File "/usr/lib/python3.9/site-packages/ipahealthcheck/core/plugin.py", line 18, in wrapper\n for result in f(*args, **kwds):\n File "/usr/lib/python3.9/site-packages/ipahealthcheck/core/files.py", line 80, in check\n fowner = pwd.getpwnam(o)\nKeyError: "getpwnam(): name not found: 'systemd-resolve'"\n"
}