Fixes issue #574.

This will now return paths in the same order as list_relative_paths.

Direct CAS-to-CAS import

Closes #574

See merge request !911

As per PEP 0479 (https://www.python.org/dev/peps/pep-0479/),
StopIteration thrown in context managers are not valid starting from
Python 3.7.

source.py: don't let StopIteration propagate to silence() contextmanager

See merge request !945

Sandbox errors (like missing host tools) are dependent on the host
system and rarely on what is actually done.

It is therefore better to not cache them as they are subject to
change between two runs.

Also add test to ensure sandbox failure are not cached