Added error propagation to gateway_request function (#1233)

Co-authored-by: BrodyWanner <[email protected]>

Author: broden-wanner

jupyter_server/gateway/gateway_client.py

@@ -728,9 +728,19 @@ async def gateway_request(endpoint: str, **kwargs: ty.Any) -> HTTPResponse:
     # NOTE: We do this here since this handler is called during the server's startup and subsequent refreshes
     # of the tree view.
     except HTTPClientError as e:
+        error_reason = f"Exception while attempting to connect to Gateway server url '{GatewayClient.instance().url}'"
+        error_message = e.message
+        if e.response:
+            try:
+                error_payload = json.loads(e.response.body)
+                error_reason = error_payload.get("reason") or error_reason
+                error_message = error_payload.get("message") or error_message
+            except json.decoder.JSONDecodeError:
+                error_reason = e.response.body.decode()
+
         raise web.HTTPError(
             e.code,
-            f"Error attempting to connect to Gateway server url '{GatewayClient.instance().url}'.  "
+            f"Error from Gateway: [{error_message}] {error_reason}. "
             "Ensure gateway url is valid and the Gateway instance is running.",
         ) from e
     except ConnectionError as e: