Update build script to fail on error.

This fixes the issue in https://github.com/coreos/etcd/pull/247 where errors in the build script and test script don't cause the whole build to fail. This uses the [-e](http://pubs.opengroup.org/onlinepubs/009695399/utilities/set.html#tag_04_127_03) bash flag.

We may need to update this to support other shells or we can move to a Makefile which has fail-on-error as the default behavior.

/cc: @philips @xiangli-cmu