patch 9.0.1572: error messages are not translated

Problem:    Error messages are not translated.
Solution:   Add _().
diff --git a/src/gui_photon.c b/src/gui_photon.c
index 0eb1282..b987e9b 100644
--- a/src/gui_photon.c
+++ b/src/gui_photon.c
@@ -2956,7 +2956,7 @@
     }
 
     if (report_error)
-	semsg(e_unknown_font_str, vim_font_name);
+	semsg(_(e_unknown_font_str), vim_font_name);
 
     return FAIL;
 }