patch 9.0.1576: users may not know what to do with an internal error

Problem:    Users may not know what to do with an internal error.
Solution:   Add a translated message with instructions.
diff --git a/src/errors.h b/src/errors.h
index 88dad4b..ddaa909 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -840,7 +840,8 @@
 #endif
 EXTERN char e_pattern_too_long[]
 	INIT(= N_("E339: Pattern too long"));
-// E340 unused
+EXTERN char e_internal_error_please_report_a_bug[]
+	INIT(= N_("E340: Internal error; if you can reproduce please report a bug"));
 EXTERN char e_internal_error_lalloc_zero[]
 	INIT(= N_("E341: Internal error: lalloc(0, )"));
 EXTERN char e_out_of_memory_allocating_nr_bytes[]