Align the gpc_error types more with the code

Add a type to specify what error values that the server side
can handle from rpc method callback functions, since this
is slightly different from the errors that can be returned
on the client side.

src/grpcbox_stream.erl

@@ -31,6 +31,7 @@
               grpc_status_message/0,
               grpc_error/0,
               grpc_error_response/0,
+              grpc_callback_error/0,
               grpc_error_data/0,
               grpc_extended_error_response/0]).
 
@@ -61,15 +62,19 @@
 
 -type t() :: #state{}.
 
--type grpc_status_message() :: unicode:chardata().
+-type grpc_status_message() :: unicode:unicode_binary().
 -type grpc_status() :: 0..16.
 -type http_status() :: integer().
--type grpc_error() :: {grpc_status(), grpc_status_message()}.
+-type grpc_error() :: {binary(), % containing a grpc_status() value as text
+                       grpc_status_message()}.
 -type grpc_error_response() :: {error, grpc_error(), #{headers => map(),
                                                        trailers => #{}}} |
                                {http_error, {http_status(), unicode:chardata()}, #{headers => map(),
                                                                                    trailers => #{}}} |
                                {error, term()}.
+-type grpc_callback_error() :: {error, grpc_error(), #{headers => map(),
+                                                       trailers => #{}}}.
+
 -type grpc_error_data() :: #{
     status := grpc_status(),
     message := grpc_status_message(),