Backend service errors result in status other than INTERNAL from frontend services

[SanjayVas]

If a frontend service calls a backend service as part of a method implementation, any error from that backend service should result in an INTERNAL gRPC status. This is standard practice for gRPC frameworks such as those used within Google.

Consider DEADLINE_EXCEEDED as an example. Any DEADLINE_EXCEEDED from a backend service should always be considered unexpected. Due to Deadline Propagation, the only time a frontend will receive a DEADLINE_EXCEEDED from a backend is if the backend deadline is too short.