Raise an error if google.colab.files.download can't find a file.
Raise an error if google.colab.files.download can't find a file.

I think this is the source of some user confusion with downloading files --
I believe this fixes #34, though that's partly guesswork.

RELNOTES: n/a

Build log: https://console.cloud.google.com/gcr/builds/8de26573-1b21-44bf-8780-f5fbb945ec3a?project=colab-datalab
Pushed docker image: gcr.io/colab-datalab/datalab:auto-188926312-2018-03-13T14-46-24-096871427-07-00

PiperOrigin-RevId: 188943071

googlecolab/colabtools,googlecolab/colabtools