kpatch-build: download correct source for the current kernel (for ubuntu)

As per issue #239.
kpatch-build: add error message if dpkg -x fails to extract source
kpatch-build: redirect output of wget, dpkg, and tar to log file, add error message if tar fails
Merge pull request #240 from flaming-toast/issue239

kpatch-build: download correct source for the current kernel (for ubuntu)