rm partial file upon non-zero exit of ExternalDownloader

we have to make sure to remove the partial file upon a non-zero exit code from the downloader..

let's take wget as an example if the user has a Good connection when downloading the Coursera class details and
has a Bad Connection when donwloading the list of lectures, the wget which has a -O option will create a blank file and exits with a read timeout error or a could not resolve error without deleting the blank file.
But that is just a single case where i faced the problem.