[Review] Request from 'aduffeck' @ 'SUSE/machinery/show_extracted_state'

Please review the following changes:
  * 3bd174f Improve machinery output in "machinery upgrade-format"
  * e8ee7f6 Write information about description upgrades to the machinery log
  * 6647cf4 Handle upgrade failures and continue in "machinery upgrade-format --all"
  * edc0cc2 DRY incompatible description errors into SystemDescriptionIncompatible
  * a325ddd Show the "extracted" state of the file scopes in "machinery show"
  * 9981fde Always show the extracted state in "machinery list"
  * b21419a Add "SystemDescription.scope_extractable?"
  * d7cfb85 Allow for filling the test descriptions with scope content
  * 7f7beca Merge pull request #260 from SUSE/add_ruby-devel_as_13.1_requirement
  * a9fdc1b Add rubygem-devel as explicit requirement for 13.1
  * 62373ad Merge branch 'remove_obsolete_variable'
  * 3666dc3 Merge pull request #253 from SUSE/review_140930_saner_dependencies_for_machinery_package
  * 7d39a45 Removed RPM dependencies to kiwi and python-glanceclient
  * 1303313 Remove obsolete class variable
What is the benefit of making this a nested condition? Shouldn't it be clear enough if `SystemDescription.scope_extracted?` returns false for scopes which can't be extracted?