session: Check for errors from g_poll()

If the call to g_poll() in sr_session_iteration() fails, report
the error back to the caller. Do not treat EINTR as error though.

Check for session abort only if a source callback was actually
invoked, or at least once if none of the callbacks are invoked.
Stop checking for abort if the session has already been stopped,
just in case a callback sets abort_session again.

Also change the documentation to match the actual behavior.