updater: Minor improvements to error handling

In RequestsFetcher it makes sense to raise "from e" but in
updater the error we raise is not related to the original
error: use "from None".

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

tuf/ngclient/_internal/requests_fetcher.py

@@ -94,7 +94,7 @@ def _fetch(self, url: str) -> Iterator[bytes]:
         except requests.HTTPError as e:
             response.close()
             status = e.response.status_code
-            raise exceptions.DownloadHTTPError(str(e), status)
+            raise exceptions.DownloadHTTPError(str(e), status) from e
 
         return self._chunks(response)
 

tuf/ngclient/updater.py

@@ -397,7 +397,7 @@ def _load_targets(self, role: str, parent_role: str) -> Targets:
             if metainfo is None:
                 raise exceptions.RepositoryError(
                     f"Role {role} was delegated but is not part of snapshot"
-                )
+                ) from None
 
             length = metainfo.length or self.config.targets_max_length
             version = None