patch 9.0.1673: cannot produce a status 418 or 503 message

Problem:    Cannot produce a status 418 or 503 message.
Solution:   Add err_teapot().
diff --git a/src/errors.h b/src/errors.h
index a209413..9b40cb2 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -1037,6 +1037,8 @@
 	INIT(= N_("E417: Missing argument: %s"));
 EXTERN char e_illegal_value_str[]
 	INIT(= N_("E418: Illegal value: %s"));
+EXTERN char e_im_a_teapot[]
+	INIT(= N_("E418: I'm a teapot"));
 EXTERN char e_fg_color_unknown[]
 	INIT(= N_("E419: FG color unknown"));
 EXTERN char e_bg_color_unknown[]
@@ -1270,6 +1272,8 @@
 	INIT(= N_("is not a file or writable device"));
 EXTERN char e_str_is_not_file_or_writable_device[]
 	INIT(= N_("E503: \"%s\" is not a file or writable device"));
+EXTERN char e_coffee_currently_not_available[]
+	INIT(= N_("E503: Coffee is currently not available"));
 	// E504
 EXTERN char e_is_read_only_cannot_override_W_in_cpoptions[]
 	INIT(= N_("is read-only (cannot override: \"W\" in 'cpoptions')"));