xen: Fix build break

python fetch_packages.py breaks in xen builder as xen doesn't have npm tool. This is not even
required in xen builder. Hence adding fix to catch OSError (thrown if it can't find npm) and
bailout gracefully.

Signed-off-by: Anirban Chakraborty <abchak@juniper.net>
Merge pull request #24 from anirban-c/master

xen: Fix build break