Fail installing pre-requisites if apt-get fails.

I had this happen to me and the script silently failed and continued
with building libevent and other things. Yet I couldn't run gyp afterwards.

Change-Id: I302b9de29843995cbeece264313c6e20fbd4b741
Reviewed-on: https://weave-review.googlesource.com/1097
Reviewed-by: Vitaly Buka <vitalybuka@google.com>