fix(subgraph_service): when the subgraph connection is closed or in error, return a proper subgraph response

[bnjjj]

When the subgraph connection is closed or in error, return a proper subgraph response containing an error. This was preventing subgraph response service to be triggered in coprocessor and rhai.

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[Geal]

this looks ok, but I wonder if we should instead have a wrapper in Service::call to convert Err to a subgraph response every time, to make sure we won't see that issue again. Or maybe make sure that call_single_http and others do not return a Result but a graphql response

[bnjjj]

@Geal IMHO there is still bad things that can happen and should still appear in map_err, here the situation is different because it's directly related to the subgraph call. But we could have a critical internal error that should be treated as a real error and so IMHO we should keep Result anywhere else.

[Geal]

yup. as long as everything that gets out of the subgraph service is a graphql response, it will be fine

[garypen]

Looks good.