try to warn about ffi startup failures

[jcupitt]

FFI can fail with RuntimeError if write-execute memory protection is on. Try to warn about this.

see #439

thanks @paulschreiber

[jcupitt]

I just used STDERR.puts for the error, since it occurs before the logger is built. Hopefully that'll be visible.

[kleisauke]

To appease standardrb, I think it should use warn instead.
https://rubystyle.guide/#warn