To better report issues in the absence of a suitable bwrap, or the
FUSE devices, this refactors the checks for sandboxing in the Linux
platform to cover the various possibilities.  The reasons are then
collated and passed to the dummy sandbox for later reporting to the
user if a local build is attempted.

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

On some potentially broken systems, running `bwrap --version` might
fail with an error code.  This patch corrects the oversight ensuring
that we cleanly return False for version checking in such cases.

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

Since there are use-cases where BuildStream could be installed
onto systems which do not have BubbleWrap (e.g. for remote-build-only
scenarios) it is not correct to assert a dependency on bwrap during
installation.  This patch makes the assertion a warning, and also
clarifies the message somewhat.

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