sandbox/_sandboxdummy.py: Take a reason for use

Since the SandboxDummy sandbox is only used in very specific
circumstances and with good reason, this adjusts the SandboxDummy
class to take a reason and to return it if `.run()` has to
raise an exception because this is a dummy sandbox.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>