Clarify missing file error (#384)

gpt_engineer/db.py

@@ -18,7 +18,7 @@ class DB:
         full_path = self.path / key
 
         if not full_path.is_file():
-            raise KeyError(key)
+            raise KeyError(f"File '{key}' could not be found in '{self.path}'")
         with full_path.open("r", encoding="utf-8") as f:
             return f.read()