Descriptors not supported by uv cause a crash

[mzabaluev]

Currently libuv aborts when EPOLL_CTL_ADD fails to add a descriptor to the poll. This can happen when the descriptor is not pollable accordingly to the kernel. The GMainContext API in GLib, however, supports any Unix descriptors because the upstream implementation uses POSIX poll().
To fix this, libuv should be changed to gracefully report such failures, e.g. by calling the poll callback once with an error status.