fix: improve source error message

[shouya]

Previously, if an endpoint failed to fetch the source, the endpoint would respond with a status 500 and an error message too specific. For example, it could be a "500: bad url" which is not very clear to the user that it's the source url that's broken or an url elsewhere.

This commit improves on the error message by providing a more user-friendly message:

• http request errors from anywhere in the pipeline will show as a 502 error with the failed status code and the requesting url is returned in the error message
• source errors will indicate it's a source error