default prompt: Set fish_color_status if unset

Otherwise this'll error with --no-config

share/functions/fish_prompt.fish

@@ -5,6 +5,8 @@ function fish_prompt --description 'Write out the prompt'
     set -l last_pipestatus $pipestatus
     set -lx __fish_last_status $status # Export for __fish_print_pipestatus.
     set -l normal (set_color normal)
+    set -q fish_color_status
+    or set -g fish_color_status --background=red white
 
     # Color the prompt differently when we're root
     set -l color_cwd $fish_color_cwd