(#8704) Give better errors for invalid fileserver.conf

If you tried to just put an allow or deny line in the fileserver.conf
without a mount point, you got a really confusing error message:

lib/puppet/network/handler/fileserver.rb:285:in `readconfig': undefined method `info' for nil:NilClass (NoMethodError)

Now instead we give an error saying no mount point was specified.

Reviewed-by: Josh Cooper <josh@puppetlabs.com>