Allow for filling the test descriptions with scope content

The system description factory now has two new options. The scopes given
in the "scopes" options will be added to the system description with
some example content. The scopes in the "extracted_scopes" option will
additionally be marked as "extracted" and the according files will be
written to disk if "store_on_disk" is true.
Add "SystemDescription.scope_extractable?"

The method returns true when the given scope is extractable, e.g. when
it has an "--extract" option that will make machinery extract files from
the inspected system.
Always show the extracted state in "machinery list"
Show the "extracted" state of the file scopes in "machinery show"

Fixes #254
DRY incompatible description errors into SystemDescriptionIncompatible

The same error message was used several times across the source code. It
is now extracted into a specific exception class for that case.

Fixes #257
Handle upgrade failures and continue in "machinery upgrade-format --all"

Fixes #256
Fixes #258
Write information about description upgrades to the machinery log

Fixes #259
Improve machinery output in "machinery upgrade-format"
Add "ScopeMixin#scope_name" method to make scopes aware of their names
"ScopeMixin#is_extractable?" returns wether a scope is extractable

"Extractable" scopes are scopes which optionally extract files from the
inspected system.
Test that the renderer adds the extraction state to scope headers
Merge pull request #263 from SUSE/show_extracted_state

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