Check the return value from uv_async_send()

Check the return value from uv_async_send()

An omission pointed out by the following report from Coverity:

    /lib/isc/loop.c: 483 in isc_loopmgr_pause()
    >>>     CID 455002:  Error handling issues  (CHECKED_RETURN)
    >>>     Calling "uv_async_send" without checking return value (as is done elsewhere 5 out of 6 times).
    483     		uv_async_send(&loop->pause_trigger);