Add logging of error message when asynchronous task fails in AsyncExecutor. Fixes #265.
Merge branch 'wip-265-table-writer-error-reporting' into wip-265-table-writer-error-reporting-b1.1
Merge pull request #305 from datastax/wip-265-table-writer-error-reporting-b1.1

Add logging of error message when asynchronous task fails in AsyncExecutor. Fixes #265.