Revert "agnoster theme shows error code instead of an "x""

This reverts commit f09af57179.

It is far more aesthetically pleasing, and conveys the correct amount of information. The exact numeric status code is extremely rarely of use, and increases cognitive parsing burden on the user.
(cherry picked from commit 8f664edcaf55adcc540f0fe43d4205b66e4f9fb6)

themes/agnoster.zsh-theme

@@ -95,7 +95,7 @@ prompt_dir() {
 prompt_status() {
   local symbols
   symbols=()
-  [[ $RETVAL -ne 0 ]] && symbols+="%{%F{red}%}$RETVAL"
+  [[ $RETVAL -ne 0 ]] && symbols+="%{%F{red}%}✘"
   [[ $UID -eq 0 ]] && symbols+="%{%F{yellow}%}⚡"
   [[ $(jobs -l | wc -l) -gt 0 ]] && symbols+="%{%F{cyan}%}⚙"