Report invalid config files instead of crashing

Invalid values (e.g. an empty or non-numeric 'keep') and syntax
errors in the config file raised uncaught exceptions. Catch them
and exit with code 6, including the reason in the error message.

simple_backup/simple_backup.py

@@ -922,8 +922,9 @@ def simple_backup() -> int:
 
     try:
         config_args = _read_config(config_file, user)
-    except (configparser.NoSectionError, configparser.NoOptionError):
+    except (configparser.Error, ValueError) as e:
-        logger.critical('Bad configuration file')
+        # configparser.Error: missing sections/options or bad syntax, ValueError: invalid values (e.g. keep=abc)
+        logger.critical('Bad configuration file: %s', e)
 
         return 6