Element: Use cached buildtree in build shells and failure shells

This includes changes in app.py:
* Interactive failure shell no longer uses the failed build sysroot,
  defaulting to the cached build tree.
* Always allow producing a failure shell.

Changes in element.py are:
* Errors caused by building don't store the failed build sysroot.
* When staging sources, will stage the element's cached build tree if it
  exists.

This fixes part of #539