* Adding file validation for download and unzip.

Refactored the download retry code.

Added error handlers for read and write streams.

WriteStream writes to a *.tmp file and then renames. This catches the
case if VSCode dies in the middle of this process and the file is
partially written. We will continue where we left off.

* Adding return to resolve/reject calls.

* Fix retry loop and check err in unlink and rename

* Actually wait for those promises

* Fix TSLint error