Adds File existence check to Uploader (#4417)

This handles the TODO for checking the existence of the files being
uploaded by the Uploader class.

https://github.com/python-poetry/poetry/blob/c967a4a5abc6a0edd29c57eca307894f6e1c4f16/poetry/publishing/uploader.py#L229


In the case that a file does not exist, it raises a UploadError.

The raised error message is as follows: `Archive ([FILENAME]) does not exist`

Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>