fix(retrieval): name the link when process/web cannot fetch it (#31351)

A link whose host refuses the connection, such as a closed port, still came back from POST /api/v1/retrieval/process/web as "Error querying knowledge base", so the caller was told the knowledge base failed when the link was the problem.

The web loaders log a failed fetch and return no documents. That empty result then failed while being saved to the vector store, and the save error was the one reported.

process_web now answers with the existing "Could not read content from <url>" 400 as soon as the loader returns no documents, the same message a link refused by the fetch filter already gets. The check sits in the endpoint so web search and the other users of the loaders keep their current behaviour.

With process=false or embedding bypassed, an unreachable link now gets the same 400 where it used to return 200 with empty content.

Fixes #31347

backend/open_webui/routers/retrieval.py

@@ -2457,6 +2457,13 @@ async def process_web(
             detail=ERROR_MESSAGES.DEFAULT(e, f'Could not read content from {form_data.url}'),
         )
 
+    # web loaders swallow fetch errors and return no documents
+    if not docs:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=ERROR_MESSAGES.DEFAULT(f'Could not read content from {form_data.url}'),
+        )
+
     try:
         log.debug('text_content: %s', content)