Improve error message for malformed pyproject.toml files

Including the file name is enough to let the user know what the problem is.

The same is not needed for `.ini` files because the error message includes the path to the file by default.

Fix #9730

src/_pytest/config/findpaths.py

@@ -70,7 +70,7 @@ def load_config_dict_from_file(
         try:
             config = tomli.loads(toml_text)
         except tomli.TOMLDecodeError as exc:
-            raise UsageError(str(exc)) from exc
+            raise UsageError(f"{filepath}: {exc}") from exc
 
         result = config.get("tool", {}).get("pytest", {}).get("ini_options", None)
         if result is not None: