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.

Changes in element.py are:
* Errors caused by building don't store the failed build sysroot,
  instead storing that a sandbox can be created to debug the error.
* When staging sources, will stage the element's cached build tree if it
  exists.

Changes in _exceptions.py:
* BstError.sandbox is now a flag of whether a sandbox can be opened up
  to debug the error.