Fail gracefully with HTTP errors

It doesn't appear to be unusual for the IntChron servers to wobble when hitting it with the volume of requests `intchron()` queries can generate. These shouldn't cause fatal errors (which, if nothing else, stop vignettes and examples building and R CMD check passing). The functions should attempt to recover, e.g. by waiting a while and retrying, and as a last resort return what data they could get with a warning.