Script fails to exit when a file download fails and produces a bad environment

[TrevorKarjanis]

Line 227 in dev_env.ps1 failed to download SetupTools and line 178 in dev_env_salt.ps1 of DownloadFileWithProgress doesn't check $Event (AsyncCompletedEventArgs) for an error. This resulted in a cascade of failed installations that require SetupTools and produced a bad environment. Should DownloadFileWithProgress check for failure and throw? Should the script throw if pip and easy_install processes exit with a failure? At the least, it should log download errors.