improvement(best-orders): return an rpc error when we can't find best orders

[mariocynicys]

Return an RPC error in best orders rpc if we were not able to get the best orders because nobody in the network replied to our p2p request.
This is saner that returning a successful result with no best orders inside.

Kinda addresses #2228

[mariocynicys]

in progress since i will check also other p2p interacting RPCs and apply the same patch to.
p2p interactions aren't guaranteed to succeed, so we should make that obvious/visible from the RPC side.

[onur-ozkan]

Since it's been a month, can you share the status quo of this PR?

Changes seem harmless, so the current status is also LGTM.

[mariocynicys]

@onur-ozkan there is one last place we ask for any peer's response but we don't error.
i opted for not erroring here since part of the response is computed from local data and isn't completely dependent p2p network.