Fix discrepancy between status names

[ocelotl]

Fixes #384

[arminru]

I think the current names are better, as InternalError and UnknownError are indeed meant to indicate an internal resp. unknown error and not any arbitrary unknown (or internal?) status.

[tedsuo]

For clarity - the issue is that our names slightly diverge from gRPC status codes, correct? In gRPC, these codes are called UKNOWN and INTERNAL.

gRPC codes: https://github.com/grpc/grpc/blob/master/doc/statuscodes.md

@arminru I do agree that UnknownError and InternalError are clearer.

@bogdandrutu as our resident gRPC expert, how much does this bikeshed matter to you? :)

[c24t]

In either case we should avoid using status code names that are nearly identical to gRPC's. I can imagine semantically-identical but differently-named status codes causing problems for people using both gRPC and OT, and that distinction being more confusing than whether these codes are named *Error.