added exception for invalid column names
added exception for invalid column names

Avinash9/pandasql,yhat/pandasql,airtoxin/pysqldf,stephanieleevillanueva/pandasql,stonebig/pandasql,yhat/pandasql,stephanieleevillanueva/pandasql,airtoxin/pysqldf,Avinash9/pandasql,stonebig/pandasql