gRPC handler errors aren't returned correctly

[srerickson]

Two separate issues, but I'll track them together:

• In grpc handlers, I'm returning naked err values on error, but they should be wrapped with grpc error codes. See here: https://connect.build/docs/go/errors
• If the GetContent() handler returns an error, the client doesn't catch it. The fix is to check the response error after receive is done.