Scanner throws exception from background thread.

[keith-turner]

While working on #718, an IT was iterating over a scanner that was created from a closed AccumuloClient. This caused the following code to throw an exception from a background thread.

https://github.com/apache/accumulo/blob/rel/1.9.2/core/src/main/java/org/apache/accumulo/core/client/impl/ScannerIterator.java#L162

Since the exception was from a background thread, I had no idea what code in the IT was using the scanner. I tried changing the code to wrap the exception from the background thread, but that broke other code looking for specific exceptions like IsolationException.

Need to do something to improve debugging while not breaking anything.