realm: Issue fatal error when pinning GPU memory with NIC fails
Per the discussion in https://github.com/StanfordLegion/legion/issues/1681, we are making it a fatal error when Realm is unable to pin memory with the GPU. This avoids failure modes where Realm continues as if this operation has succeeded but it actually has not, leading to crashes later on.
The version in this MR has been modified to be compatible with all known GASNet versions.