l3build: Halt CTAN zip creation if docs fail to build

This happens reasonably often with source3, so should be picked up.

Question: Do we want to force checks to be run as part of teh ctan target?
Currently we don't, and perhaps it's more sensible to expect that to be done
by hand (which is my procedure now).


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5115 de43f980-851b-0410-b2f7-c40aca1f87e0