raise URLError when CachedWeb gets non-200 code
raise URLError when CachedWeb gets non-200 code

This is to stop bad responses from being put into the python cache.

(sapling split of c00ac092f285d5a71fc92c494b1890b6b89b43fc)
snyaggarwal/pex,snyaggarwal/pex,rouge8/pex,sixninetynine/pex,sixninetynine/pex,kwlzn/pex,URXtech/pex,kwlzn/pex,pantsbuild/pex,rouge8/pex,jamesbroadhead/pex,pombredanne/pex,URXtech/pex,jsirois/pex,mzdanieltest/pex,pombredanne/pex,jamesbroadhead/pex,pantsbuild/pex,jsirois/pex,mzdanieltest/pex