scripts/graph-build-time: properly warn about missing modules

Currently the graph-build-time script prints a python exception if a
needed module cannot be imported. Catch the exception and tell the user
which packages are missing, as we do for other missing dependencies.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>