Create a pattern for reporting fine-grained error details in a machine-readable way

Currently, we are testing the error message strings emitted from ate-api-server in functional_test.go. This is pretty fraught, since those messages aren't necessarily going to be stable as code is refactored.

Instead, we should use the [Status.Details field](https://pkg.go.dev/google.golang.org/grpc/status#FromProto) with an ate-specific details proto. We can define a fine-grained code for each logical error, in case clients need to distinguish situations beyond just the canonical code.

For example,
* If we can't find a free worker, we might return a status error where code=RESOURCE_EXHAUSTED, and details has an ateapipb.ErrorDetails proto with fine_grained_code=NO_FREE_WORKERS