Fix error message on missing python
Fix error message on missing python

mixminion/mixminion,rxcomm/mixminion,Javex/mixminion,Javex/mixminion,mixminion/mixminion,Javex/mixminion,nmathewson/mixminion,mixminion/mixminion,nmathewson/mixminion,nmathewson/mixminion,crooks/mixminion,Javex/mixminion,reezer/mixminion,reezer/mixminion,reezer/mixminion,crooks/mixminion,crooks/mixminion,rxcomm/mixminion,rxcomm/mixminion,reezer/mixminion,nmathewson/mixminion,crooks/mixminion,rxcomm/mixminion,mixminion/mixminion