fix(config): Raise proper error for missing config file

When user supplies invalid config files setting it causes `anyconfig` to
raise problem with parsing `yaml` file - which is misleading.

Fix by verification of existence of config files before proceeding to
anyconfig.

fixes: #8844

sdcm/sct_config.py

@@ -1766,6 +1766,9 @@ def __init__(self):  # noqa: PLR0912, PLR0914, PLR0915
 
         # 2) load the config files
         try:
+            for conf_file in list(config_files):
+                if not os.path.exists(conf_file):
+                    raise FileNotFoundError(f"Couldn't find config file: {conf_file}")
             files = anyconfig.load(list(config_files))
             anyconfig.merge(self, files)
         except ValueError: