agnoster: add guard to bzr prompt

Depending on the exit code of 'bzr status' is not enough,
as some command_not_found_handler could be used which
would then override that code.
Use the associative list of commands from zsh/param instead.

Signed-off-by: Olivier Tilmans <olivier.tilmans@uclouvain.be>

themes/agnoster.zsh-theme

@@ -128,6 +128,7 @@ prompt_git() {
 }
 
 prompt_bzr() {
+    (( $+commands[bzr] )) || return
     if (bzr status >/dev/null 2>&1); then
         status_mod=`bzr status | head -n1 | grep "modified" | wc -m`
         status_all=`bzr status | head -n1 | wc -m`