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

This is related to #539

This test checks that the interactive failure shell stages the cached
build tree.

i.e. instead of taking a directory that must contain "root" and
"scratch", and treating "root" as the root, use the directory directly.

In element.py:
* __sandbox takes the `bare_sandbox` arg, to pass into the sandbox's constructor

In sandbox.py:
* If bare_sandbox, `_root` is the passed-in directory, and `__scratch`
  is None.
* Trying to use `__scratch` when bare_sandbox is True is a bug.

In _mount.py:
* Don't get the value of `__scratch` if it's not needed.

This is part of #539

This is related to #539