GetWordGraph errors

We've seen a handful of this error on production:

Grpc.Core.RpcException: Status(StatusCode="Unknown", Detail="Exception was thrown by handler.")
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task, ConfigureAwaitOptions options)
   at System.Runtime.CompilerServices.TaskAwaiter<TResult>.GetResult()
   at Serval.Translation.Services.EngineService+<GetWordGraphAsync>d__13.MoveNext() in /app/src/Serval/src/Serval.Translation/Services/EngineService.cs:line 112
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task, ConfigureAwaitOptions options)
   at System.Runtime.CompilerServices.TaskAwaiter<TResult>.GetResult()
   at Serval.Translation.Controllers.TranslationEnginesController+<GetWordGraphAsync>d__16.MoveNext() in /app/src/Serval/src/Serval.Translation/Controllers/TranslationEnginesController.cs:line 313

on the machine end:

Grpc.AspNetCore.Server.ServerCallHandler[7]
      Error status code 'Aborted' with detail 'The engine must be built first.' raised.
      Serval.Machine.Shared.Utils.EngineNotBuiltException: The engine must be built first.
         at Serval.Machine.Shared.Services.SmtTransferEngineService.GetBuiltEngineAsync(String engineId, CancellationToken cancellationToken) in /app/src/Machine/src/Serval.Machine.Shared/Services/SmtTransferEngineService.cs:line 288
         at Serval.Machine.Shared.Services.SmtTransferEngineService.GetWordGraphAsync(String engineId, String segment, CancellationToken cancellationToken) in /app/src/Machine/src/Serval.Machine.Shared/Services/SmtTransferEngineService.cs:line 141
         at Serval.Machine.Shared.Services.ServalTranslationEngineServiceV1.GetWordGraph(GetWordGraphRequest request, ServerCallContext context) in /app/src/Machine/src/Serval.Machine.Shared/Services/ServalTranslationEngineServiceV1.cs:line 78

Start with GetWordGraphAsync in Serval.Translation/Services/EngineService.cs and TranslationEnginesController.cs, then trace the machine-side calls in SmtTransferEngineService.cs and ServalTranslationEngineServiceV1.cs at the reported lines. Determine the expected behavior when the engine is not built, then add coverage showing that GetWordGraph handles that state correctly.