Throw an error message when $WORKON_HOME is not set

Took me a while to figure this one out, and I have a default installation of virtualenvwrapper -- this is a soft fix, just put an error message. But perhaps the fix should be to use the default value `~/.virtualenvs`.

plugins/virtualenvwrapper/virtualenvwrapper.plugin.zsh

 virtualenvwrapper='virtualenvwrapper.sh'
 if (( $+commands[$virtualenvwrapper] )); then
+
   source ${${virtualenvwrapper}:c}
 
+  if [[ "$WORKON_HOME" == "" ]]; then
+    echo "\$WORKON_HOME is not defined so ZSH plugin virtualenvwrapper will not work"
+  else
+
     if [[ ! $DISABLE_VENV_CD -eq 1 ]]; then
       # Automatically activate Git projects's virtual environments based on the
       # directory name of the project. Virtual environment name can be overridden
@@ -54,6 +59,7 @@ if (( $+commands[$virtualenvwrapper] )); then
         set -A chpwd_functions workon_cwd
       fi
     fi
+  fi
 else
   print "zsh virtualenvwrapper plugin: Cannot find ${virtualenvwrapper}. Please install with \`pip install virtualenvwrapper\`."
 fi