Make not having setuptools or pip sdists a fatal error

Makes it a fatal error for the setuptools and/or pip sdists to be missing from the virtualenv support directory.

Review requested - I am not completely convinced this is a good idea, as it changes the current behaviour, where the resulting virtualenv is usable but missing pip/setuptools (which can then be installed manually) to leave the new virtualenv incomplete (the activate scripts are not installed, for example).

Having an incomplete/missing virtualenv_support directory is unsupported in any case, so this should fall firmly into the "cannot happen" area, anyway...