Fix exception causes in server.py

Author: cool-RR

src/websockets/server.py

@@ -340,8 +340,8 @@ def process_origin(
         # per https://tools.ietf.org/html/rfc6454#section-7.3.
         try:
             origin = cast(Origin, headers.get("Origin"))
-        except MultipleValuesError:
-            raise InvalidHeader("Origin", "more than one Origin header found")
+        except MultipleValuesError as e:
+            raise InvalidHeader("Origin", "more than one Origin header found") from e
         if origins is not None:
             if origin not in origins:
                 raise InvalidOrigin(origin)