MM-10232, MM-10259: Improve error handling from invalid json (#8668)

* MM-10232: improve error handling from malformed slash command responses

Switch to json.Unmarshal, which doesn't obscure JSON parse failures like
json.Decode. The latter is primarily designed for streams of JSON, not
necessarily unmarshalling just a single object.

* rework HumanizedJsonError to expose Line and Character discretely

* MM-10259: pinpoint line and character where json config error occurs

* tweak HumanizeJsonError to accept err first

app/command.go

@@ -246,8 +246,9 @@ func (a *App) ExecuteCommand(args *model.CommandArgs) (*model.CommandResponse, *
 					return nil, model.NewAppError("command", "api.command.execute_command.failed.app_error", map[string]interface{}{"Trigger": trigger}, err.Error(), http.StatusInternalServerError)
 				} else {
 					if resp.StatusCode == http.StatusOK {
-						response := model.CommandResponseFromHTTPBody(resp.Header.Get("Content-Type"), resp.Body)
-						if response == nil {
+						if response, err := model.CommandResponseFromHTTPBody(resp.Header.Get("Content-Type"), resp.Body); err != nil {
+							return nil, model.NewAppError("command", "api.command.execute_command.failed.app_error", map[string]interface{}{"Trigger": trigger}, err.Error(), http.StatusInternalServerError)
+						} else if response == nil {
 							return nil, model.NewAppError("command", "api.command.execute_command.failed_empty.app_error", map[string]interface{}{"Trigger": trigger}, "", http.StatusInternalServerError)
 						} else {
 							return a.HandleCommandResponse(cmd, args, response, false)