patch 8.2.4919: can add invalid bytes with :spellgood

Problem:    Can add invalid bytes with :spellgood.
Solution:   Check for a valid word string.
diff --git a/src/errors.h b/src/errors.h
index 1e9be6c..087f497 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3273,3 +3273,7 @@
 EXTERN char e_missing_close_curly_str[]
 	INIT(= N_("E1279: Missing '}': %s"));
 #endif
+#ifdef FEAT_SPELL
+EXTERN char e_illegal_character_in_word[]
+	INIT(= N_("E1280: Illegal character in word"));
+#endif