patch 8.2.2173: Vim9: get internal error when assigning to undefined variable

Problem:    Vim9: get internal error when assigning to undefined variable.
Solution:   Add error message. (closes #7475)
diff --git a/src/errors.h b/src/errors.h
index e761a4e..023c1d1 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -321,3 +321,5 @@
 	INIT(= N_("E1144: Command is not followed by white space: %s"));
 EXTERN char e_missing_heredoc_end_marker_str[]
 	INIT(= N_("E1145: Missing heredoc end marker: %s"));
+EXTERN char e_command_not_recognized_str[]
+	INIT(= N_("E1146: Command not recognized: %s"));