Surface more debugging information when spec URLs fail on retrieval

[erunion]

https://github.com/readmeio/api/blob/main/packages/api/src/fetcher.ts#L49-L54

When you pass in a URL that 404s we just tell you that the URL was not found which isn't supper helpful if the URL you're supplying is doing redirects behind the scenes (like in the case of accessing a /openapi URL for a private project on ReadMe redirects to /404).