create: define the error code when creating a resource fails.

[toumorokoshi]

aep.dev/133 does not specify what error to return back, if it cannot create the resource.

Perhaps INVALID_ARGUMENT would be best in the common case?

  // The client specified an invalid argument.  Note that this differs
  // from `FAILED_PRECONDITION`.  `INVALID_ARGUMENT` indicates arguments
  // that are problematic regardless of the state of the system
  // (e.g., a malformed file name).
  //
  // HTTP Mapping: 400 Bad Request
  INVALID_ARGUMENT = 3;