api/msgpack-rpc: Improve error infrastructure

- Add error type information to `Error`
- Rename `set_api_error` to `api_set_error` for consistency with other api_*
  functions/macros.
- Refactor the api_set_error macro to accept formatted strings and error types
- Improve error messages
- Wrap error messages with gettext macro
- Refactor msgpack-rpc serialization to transform Error instances into [type,
  message] arrays
- Add error type information to API metadata
- Normalize nvim->client and client->nvim error handling(change
  channel_send_call to accept an Error pointer instead of the `errored` boolean
  pointer)
- Use macro to initialize Error structures

src/nvim/eval.c

@@ -12471,25 +12471,18 @@ static void f_rpcrequest(typval_T *argvars, typval_T *rettv)
     ADD(args, vim_to_object(tv));
   }
 
-  bool errored;
-  Object result;
-  if (!channel_send_call((uint64_t)argvars[0].vval.v_number,
-                         (char *)argvars[1].vval.v_string,
-                         args,
-                         &result,
-                         &errored)) {
-    EMSG2(_(e_invarg2), "Channel doesn't exist");
-    return;
-  }
-
-  if (errored) {
-    vim_report_error(result.data.string);
+  Error err = ERROR_INIT;
+  Object result = channel_send_call((uint64_t)argvars[0].vval.v_number,
+                                    (char *)argvars[1].vval.v_string,
+                                    args,
+                                    &err);
+  if (err.set) {
+    vim_report_error(cstr_as_string(err.msg));
     goto end;
   }
-  
-  Error conversion_error = {.set = false};
-  if (!object_to_vim(result, rettv, &conversion_error)) {
-    EMSG(_("Error converting the call result"));
+
+  if (!object_to_vim(result, rettv, &err)) {
+    EMSG2(_("Error converting the call result: %s"), err.msg);
   }
 
 end:
@@ -19442,7 +19435,7 @@ static void script_host_eval(char *method, typval_T *argvars, typval_T *rettv)
     return;
   }
 
-  Error err = {.set = false};
+  Error err = ERROR_INIT;
   object_to_vim(result, rettv, &err);
   api_free_object(result);