Use Popen.communicate instead of subprocess call

so we can caputure stderr and pass it along with any errors.
Revert "Use Popen.communicate instead of subprocess call"

This reverts commit 0c582cd827f164168a313e0bb4eabf9487bdd088.
dch: Use DebianSource.is_native

instead of ChangeLog.is_native since the former used
debian/source/format in favor of debian/changelog.
ChangeLog: Remove is_native

to avoid accidental usage. Use DebianSource.is_native instead.