Failing to set a valid MirSurfaceParameters.output_id causes cryptic exceptions in clients: Failed to import PRIME fd for DRM buffer

As discovered in bug 1215754, if you forget to or incorrectly set MirSurfaceParameters.output_id then your client will crash with a cryptic error:

terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<std::runtime_error> >'
  what(): Failed to import PRIME fd for DRM buffer

We clearly need better error checking.