[FlightRPC][C++] Some status codes don't round-trip through gRPC

[asfimport]

KeyError and AlreadyExists don't fully round-trip, instead becoming UNKNOWN. There are others, but we don't attempt to map all the Arrow status to a gRPC status, only the ones that closely correspond to a gRPC error. 

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

PRs and other links:

• GitHub Pull Request #6615

_{Note: This issue was originally created as ARROW-8112. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Is it reasonably possible to pass the Status code on the side, e.g. as a custom HTTP header? Or is that impossible/too much work?

[asfimport]

David Li / @lidavidm:
Yes, it's possible to pass custom headers @pitrou. But if we value "it looks like plain gRPC" we should try to map to and from gRPC codes IMO.

[asfimport]

Antoine Pitrou / @pitrou:
Well, we can do both: map as much as we can for third-party clients (as a best effort), but also pass a custom header for the Arrow C++ client to read from and faithfully reproduce the original error.

[asfimport]

David Li / @lidavidm:
Sure, that makes sense. Let me open a separate issue so that we can track the cross-language work.

[asfimport]

David Li / @lidavidm:
Oh, wait, no, that only makes sense in C++.

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 6615
#6615