Copies of architecture-independent binaries may fail between series with different sets of supported architectures

This problem happens today in https://edge.launchpad.net/~ubuntugis/+archive/ubuntugis-unstable when copy source and it's arch-indep binaries from jaunty to intrepid.

We retrieve the unique arch-indep binary package release for the copy and if happens to be the one published in 'armel' do_copy() will just skip the step that copies the binary to all the destination architectures.

check_copy() does the correct checks and the copy with binaries is legitimate, but the copy execution fails and the lack of binaries in the destination context triggers the creation of a illegal build record, which will certainly end up in failed-to-upload state (since the binary is already published in the archive)