131: Added toast when indexers are down

[piersss]

Description

Fixes #131

https://github.com/airswap/airswap-marketplace/assets/86911296/551219dc-d3f9-4b79-8168-137cb051e73e

Steps to test

In indexerSlice.ts change line 37 to:

urls: action.payload.map((url) => `${url}xxx`),` 

This way indexers will fail and failed toasts will get triggered.

[piersss]

Added "Unable to fetch listings" text to page.