Do not fail source retrieval step with apt >=1.1

apt >=1.1 tries to drop privileges which fails in our typical setup with:

| W: Can't drop privileges for downloading as file 'zsh_5.2-2.dsc' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

Instead let's check for actual errors (starting with "E:"),
and do not fail the build if there warnings only ("W:").