setup.py: Change bwrap assertion to a warning

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.  This should fix #644

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