``print()`` has no business being in library code like this. It was
a holdover from this code being copied from bootstrap. So remove it.
While we're here, replace the generic exception with a specific one.
We don't want to be swallowing bugs via ``except Exception``.
MozReview-Commit-ID: 49goUstfPBz
--HG--
extra : rebase_source : a821159bd12a449ed1a0edf21a1f9eb29711ad95