sources: ignore non existent files in OverrideSource

frontools/sources.py

@@ -1,6 +1,7 @@
 """Source for remote files"""
 from abc import ABC, abstractmethod
 from contextlib import asynccontextmanager
+from pathlib import Path
 from re import Pattern
 from re import compile as re_compile
 from typing import AsyncGenerator, AsyncIterable, Optional, cast
@@ -132,9 +133,10 @@ class OverrideSource(Source):
 
         for pattern, replace in self._mappings:
             if pattern.match(url):
-                mapped_path = pattern.sub(replace, url)
-                with open(mapped_path, "rb") as mapped_file:
-                    return mapped_file.read()
+                mapped_path = Path(pattern.sub(replace, url))
+                if mapped_path.is_file():
+                    with open(mapped_path, "rb") as mapped_file:
+                        return mapped_file.read()
 
         return await self._next_source.get_url(url)