patch 8.2.0816: terminal test fails when compiled with Athena

Problem:    Terminal test fails when compiled with Athena.
Solution:   Do give an error when the GUI is not running. (hint by Dominique
            Pelle, closes #5928, closes #6132)

src/gui.c

@@ -4741,7 +4741,7 @@ gui_get_color(char_u *name)
 	    && gui.in_use
 #endif
 	    )
-	semsg(_("E254: Cannot allocate color %s"), name);
+	semsg(_(e_alloc_color), name);
     return t;
 }