Fix using validators set in __manifest__.py

E.g. you can use the validator 'is_file' to check if a config is a valid file

src/auto_archiver/core/validators.py

@@ -1,7 +1,19 @@
-# used as validators for config values.
+# used as validators for config values. Should raise an exception if the value is invalid.
+from pathlib import Path
+import argparse
 
 def example_validator(value):
-    return "example" in value
+    if "example" not in value:
+        raise argparse.ArgumentTypeError(f"{value} is not a valid value for this argument")
+    return value
 
 def positive_number(value):
-    return value > 0
+    if value < 0:
+        raise argparse.ArgumentTypeError(f"{value} is not a positive number")
+    return value
+
+
+def valid_file(value):
+    if not Path(value).is_file():
+        raise argparse.ArgumentTypeError(f"File '{value}' does not exist.")
+    return value