Emit a better error message when loading the wrappers fails

[nirinchev]

If you have a multiproject solution and don't add Realm to the executable project, wrappers will fail to load with a System.DllNotFoundException. We should try and see if we can detect that and emit a more informative error message.

To repro, create a XF project and only add Realm to the shared project.

[nirinchev]

Another option proposed by @fealebenpae is to make the wrapper targets buildTransitive.