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`.
Merge pull request #3352 from benjaoming/virtualenvwrapper_workon_unset

Throw an error message when $WORKON_HOME is not set