Add RuntimeError if we fail to write pkg_resources into the .pex
Add RuntimeError if we fail to write pkg_resources into the .pex

Reviewed at https://rbcommons.com/s/twitter/r/115/

(sapling split of 6368d9ce38b91fb101e0e97865e21aff8e42d839)
pantsbuild/pex,jsirois/pex,mzdanieltest/pex,rouge8/pex,snyaggarwal/pex,URXtech/pex,jamesbroadhead/pex,sixninetynine/pex,jamesbroadhead/pex,mzdanieltest/pex,pombredanne/pex,URXtech/pex,sixninetynine/pex,kwlzn/pex,kwlzn/pex,snyaggarwal/pex,pombredanne/pex,pantsbuild/pex,rouge8/pex,jsirois/pex