patch 8.2.0990: Using duplicate error number

Problem:    Using duplicate error number.
Solution:   Use an unused error number.  Add a test for it.
diff --git a/src/globals.h b/src/globals.h
index bbc12ce..8cc9385 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1699,8 +1699,7 @@
 EXTERN char e_unknown_option[]	INIT(= N_("E113: Unknown option: %s"));
 EXTERN char e_letunexp[]	INIT(= N_("E18: Unexpected characters in :let"));
 EXTERN char e_reduceempty[]	INIT(= N_("E998: Reduce of an empty %s with no initial value"));
-// TODO: Change Error Number
-EXTERN char e_no_dict_key[]	INIT(= N_("E999: Dictionary with key \"%s\" required"));
+EXTERN char e_no_dict_key[]	INIT(= N_("E857: Dictionary key \"%s\" required"));
 #endif
 #ifdef FEAT_QUICKFIX
 EXTERN char e_readerrf[]	INIT(= N_("E47: Error while reading errorfile"));