patch 8.2.1444: error messages are spread out and names can be confusing

Problem:    Error messages are spread out and names can be confusing.
Solution:   Start moving error messages to a separate file and use clear
            names.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 98ae733..2197cca 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1457,7 +1457,7 @@
 
 		    if (dict == NULL)
 		    {
-			emsg(_(e_dictnull));
+			emsg(_(e_dictionary_not_set));
 			goto on_error;
 		    }
 		    if (key == NULL)