Distinguishing zero results because of no results, or because of server error

[dstansby]

This weekend the solar orbiter archive was down, which prompted this bug report for a not very nice error being raised by sunpy-soar: https://github.com/dstansby/sunpy-soar/issues/11. I think the solution to that issue is to return zero results from the client, so other clients have a chance to return results (e.g. for a time only search). It would be helpful to have a way of informing the user of the difference between:

• Returned zero results because the server didn't have any results
• Returned zero results because of a server error

[Cadair]

We could do better, but what some clients do is to raise warnings and then return 0 results.

[dstansby]

I am 👍 to resolving this by recommending people writing clients raise a warning if the server they are querying is down, and by putting such warnings in the sunpy core clients.

[ryuusama09]

So after doing a bit of testing , I found that res._numfile == 0 in case of 0 matched warnings . more like the len(meta_list) ==0 which essentially means the same thing. Now to reproduce a server error , I removed the part which checks that start date <= end date. Even after this , the response object was looking same like in the former case. I think the server error status should be passed by the scraper to the client