Returns false when rvm is not found.

lib/rvm.zsh

 # get the name of the ruby version
 function rvm_prompt_info() {
-  [ -f $HOME/.rvm/bin/rvm-prompt ] || return
+  [ -f $HOME/.rvm/bin/rvm-prompt ] || return 1
   local rvm_prompt
   rvm_prompt=$(eval "$HOME/.rvm/bin/rvm-prompt $ZSH_THEME_RVM_PROMPT_OPTIONS" 2>/dev/null)
-  [[ "${rvm_prompt}x" == "x" ]] && return
+  [[ "${rvm_prompt}x" == "x" ]] && return 1
   echo "${ZSH_THEME_RVM_PROMPT_PREFIX:=(}${rvm_prompt}${ZSH_THEME_RVM_PROMPT_SUFFIX:=)}"
 }