Decent overview, but, remember that when evaluating something, projects change over time. Even better, you can be the change you want to see! Thus far, the grpc project has been fairly responsive in making solid changes, either through PRs or filing issues. ;)
Regarding the complaint about errors, there is already protocol support for structured error handling: https://godoc.org/google.golang.org/grpc/status. https://github.com/grpc/grpc-go/pull/1358 should make this easier to use. In practice, the provided error code set is very good, so give them a try before making things more complex.
Worst case, you can just stuff things in a header or trailer.