Fix incorrect exception handling in KeyUploadServlet.on_POST (#7563)

Introduced in #7556

changelog.d/7563.misc

@@ -0,0 +1 @@
+Stop logging some expected HTTP request errors as exceptions.

synapse/app/generic_worker.py

@@ -214,7 +214,7 @@ class KeyUploadServlet(RestServlet):
                     self.main_uri + request.uri.decode("ascii"), body, headers=headers
                 )
             except HttpResponseException as e:
-                raise e.to_synapse() from e
+                raise e.to_synapse_error() from e
             except RequestSendFailed as e:
                 raise SynapseError(502, "Failed to talk to master") from e