Report missing plugin/unsupported file type

document.c

@@ -110,6 +110,7 @@ zathura_document_open(zathura_plugin_manager_t* plugin_manager, const char*
 
   if (plugin == NULL) {
     girara_error("unknown file type\n");
+    error = ZATHURA_ERROR_UNKNOWN;
     goto error_free;
   }
 

zathura.c

@@ -562,6 +562,9 @@ document_open(zathura_t* zathura, const char* path, const char* password,
   zathura_document_t* document = zathura_document_open(zathura->plugins.manager, path, password, &error);
 
   if (document == NULL) {
+    if (error == NULL ) {
+      girara_notify(zathura->ui.session, GIRARA_ERROR, _("Unsupported file type. Please install the necessary plugin."), false, NULL, NULL, NULL);
+    }
     if (error == ZATHURA_ERROR_INVALID_PASSWORD) {
       zathura_password_dialog_info_t* password_dialog_info = malloc(sizeof(zathura_password_dialog_info_t));
       if (password_dialog_info != NULL) {