Share Sys.time() when a task fails in `loop_tasks`

These are helpful messages: `Skipping task #2 due to error`
Even more helpful for me would be to include `Skipping task #2 due to error at 2018-06-18 17:46:52 CDT` for diagnostic purposes.

Start at the `loop_tasks` entry point and locate where the “Skipping task #2 due to error” message is assembled. Check how `Sys.time()` is formatted elsewhere, then update the failure message to include the timestamp and verify the resulting diagnostic output.

Written by the indexing model from the issue text.