Tristan/architecture docs

See merge request !907

We shouldn't need it to persist now that we cache failed build dirs.

This change breaks the test
`tests/integration/shell.py::test_sysroot_workspace_visible`.
I can no longer see a use-case for this test.
AIUI, it tested that the failed build sysroot stored in the builddir has
the workspace's files in, despite the workspace being unmounted.
I believe this behaviour is made redundant by cached buildtrees.

This fixes part of #539

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.

Changes in widget.py:
* Don't try to print any information about the sandbox.

Changes in _message.py:
* Fix documentation so Message.sandbox is not a directory any more.

This is part of #539

This is related to #539

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