There are already so many exceptions catched, like socket errors (e.g. failure
in name resolution) or HTTP errors. Depending on when a timeout occurs, it is
either catched (URLError during the SSL handshake) or not (socket.error while
getting a HTTP response).

When used by buildout, this fixes random failures when running in newest mode
(which is the default case), or when the requested version is available in the
download-cache.

easy_install has code to handle parsing a requirement, catching the
ValueError and then raising a DistUtilsError. This code was entirely
duplicated in package_index, so I've slightly refactored to remove
the duplication.