Define the error conditions for ALooper_pollOnce.

"Unrecoverable internal errors" here means that ALOOPER_POLL_ERROR
will be returned if the internal epoll_wait call fails with something
other than EINTR. I could say that explicitly, but I think the
important info for the caller is "you're doomed, just abort".

Bug: None
Test: None
Change-Id: I869f29122500ee8c71390cae1c28108a63088a77