(#14123) Puppet shouldn't explode if PATH contains ~nonexistent_user

As reported in http://bugs.debian.org/669650, when the PATH contained an
expansion for a user that doesn't exist, Puppet handled it the same way Ruby
does - to raise an exception, and explode.

Leaving aside the sanity of literal tilde in the PATH, we shouldn't explode in
that case - we should just skip that entry and carry on to the next element in
the path instead.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>