added an error message for missing column header values

I added a hopefully human readable error message when a `KeyError` is thrown. I suppose we could create a method that verifies *all* the column before parsing the key:values out. I'm willing to do that if you prefer, but this is quick and dirty and does mostly the same.

Example error message:
`KeyError: please verify 910182.csv has the value 'Labels' as a column header`