Handle situation where API abuse limit is hit

[edwardstm]

Currently, we silently handle java.io.IOException, mostly by returning null or empty objects. We should instead make use of org.kohsuke.github.AbuseLimitHandler, documented here.

[edwardstm]

Per product owner, we should implement the FAIL strategy and alert. In the future it may be desired to switch to the WAIT strategy, but still alert in case the user wishes to retry later.

[edwardstm]

Additionally implement org.kohsuke.github.RateLimitHandler's FAIL strategy for rate limit handling.

[edwardstm]

Initial implementation in #50 – testing needs discussion, and we should probably implement our own non-abstract LimitHandlers.