patch 9.0.1077: can add text property with negative ID before virtual text

Problem:    Can add text property with negative ID before virtual text
            property.
Solution:   Remember that a text property with a negative ID was used and give
            an appropriate error message. (closes #11725)
            Fix index computation.
diff --git a/src/errors.h b/src/errors.h
index 345b4ac..e33f2e6 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3397,3 +3397,7 @@
 EXTERN char e_member_not_found_on_class_str_str[]
 	INIT(= N_("E1338: Member not found on class \"%s\": %s"));
 #endif
+#ifdef FEAT_PROP_POPUP
+EXTERN char e_cannot_add_textprop_with_text_after_using_textprop_with_negative_id[]
+	INIT(= N_("E1339: Cannot add a textprop with text after using a textprop with a negative id"));
+#endif