parallel builder should make sure sources/patches are available

[boegel]

The parallel builder downloads sources before it submits jobs to perform the build (this helps avoid multiple jobs downloading the same file and creating a mess).

However, it currently doesn't check whether those downloads succeeded, and just submits the job anyway.

This should be fixed, the parallel builder should make sure all the required sources and patches are found before submitting the jobs.

This can be done by making Application.prepare_build return a boolean indicating if it's OK to proceed, and checking that return value in parallelbuild.prepare_package and parallelbuild.build_packages_in_parallel.