parallel_emerge: clean up failed summary
parallel_emerge: clean up failed summary

When multiple packages fail, the output currently looks like:
Packages failed: chromeos-base/vboot_reference-1.0-r764 ,chromeos-base/power_manager-0.0.1-r622

The spacing around the commas is incorrect, and with a lot of packages,
it is much harder scan.  Throw some newlines in there to get:
Packages failed:
	chromeos-base/vboot_reference-1.0-r764
	chromeos-base/power_manager-0.0.1-r622

BUG=None
TEST=`./build_packages` fails multiple packages and shows nice output

Change-Id: Ibf03439a516adb625d8aaddeb60059e97023c7eb
Reviewed-on: https://gerrit.chromium.org/gerrit/37148
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,coreos/chromite