patch 8.2.3961: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more errors to errors.h.
diff --git a/src/gui_x11.c b/src/gui_x11.c
index 92bbfc3..2e87177 100644
--- a/src/gui_x11.c
+++ b/src/gui_x11.c
@@ -1887,7 +1887,7 @@
     if (font == NULL)
     {
 	if (giveErrorIfMissing)
-	    semsg(_(e_font), name);
+	    semsg(_(e_unknown_font_str), name);
 	return NOFONT;
     }
 
@@ -1911,7 +1911,7 @@
 
     if (font->max_bounds.width != font->min_bounds.width)
     {
-	semsg(_(e_fontwidth), name);
+	semsg(_(e_font_str_is_not_fixed_width), name);
 	XFreeFont(gui.dpy, font);
 	return NOFONT;
     }
@@ -2072,7 +2072,7 @@
     if (fontset == NULL)
     {
 	if (giveErrorIfMissing)
-	    semsg(_(e_fontset), name);
+	    semsg(_(e_unknown_fontset_str), name);
 	return NOFONTSET;
     }