(RE-246) Make build extras optional

If the extra build data file in github.com/puppetlabs/build-data doesn't exist
for a project, it probably means we don't need it. We should avoid failing if
  its not there. This commit updates the packaging repo to do just this. Its a
  bit of an overreach, but curl returns 22 when the url isn't found or some
  other issue where a server returns 400 or above, so we'll go with it. There's
  enough curl error codes to handle most of the common network failures, that
  I'm fairly confident this is a safe move.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>