.bashrc and .zshrc backup fails when the files don't exist

[ScottG489]

This seems to happen when the backup happens, then the script fails before the new one is put into place. Then on subsequent runs the backup will fail.

The backing up seems a little dubious, although it may be alright. The right fix to this may just be to have the backup not fail if the original file doesn't exist. This is safe because the reason we do the backup is so we don't blow away the old one/stow doesn't fail to link it. So if it doesn't exist then we're all good.