[Informative VCS Prompt] Print failing exit codes

Before this change, if a command failed, this was indicated by the "$"
at the end of the prompt turning red.

With this change in place, if a command fails, the exit code of the
failing command is displayed in [square brackets].

share/tools/web_config/sample_prompts/informative_vcs.fish

@@ -90,9 +90,9 @@ function fish_prompt --description 'Write out the prompt'
 
     if not test $last_status -eq 0
         set_color $fish_color_error
+        echo -n "[$last_status] "
+        set_color normal
     end
 
     echo -n "$suffix "
-
-    set_color normal
 end