llm: Make "POST predict" error message more informative

"POST predict" basically means that the runner has crashed, which
can have many reasons. However, many people think this is a specific
error and either report only this message or group together unrelated
bugs. This replaces it with a more friendly and helpful message.

llm/server.go

@@ -798,7 +798,8 @@ func (s *llmServer) Completion(ctx context.Context, req CompletionRequest, fn fu
 
 	res, err := http.DefaultClient.Do(serverReq)
 	if err != nil {
-		return fmt.Errorf("POST predict: %v", err)
+		slog.Error("post predict", "error", err)
+		return errors.New("runner has crashed, check the log for additional detail")
 	}
 	defer res.Body.Close()