Handle errors from plugins in tablet management iterator

[keith-turner]

In #3460 the TabletManagementIterator stated calling compaction related plugins to determine it a tablet needed compaction. If these plugins throw an exception it could interfere with other manager functions like tablet assignment. Need to handle this situation in such a way that other manager functions can continue and the minimum relevant information needed for debugging is logged. Since the plugins are callled for each tablet, need to be careful about spamming the logs with lots or redundant information.

[keith-turner]

Chatted w/ @dlmarion about this. We discussed that if the TabletManagementIterator reports back an enums indicating a problem that the manager can aggregate this and emit a metric. So if the TabletManagementIterator runs on 10 tablets servers and each sees 100 errors trying to plan compactions, then the manager could emit a metric that 1000 tablets are having problems planning compactions.