When we submit a task to a thread pool for asynchronous execution, we
are returned a future. Since we submitted to go asynchronous, these
futures are not inspected for failure (we would have to block a thread
to do that). While we have on failure handlers for exceptions that are
thrown during execution, we do not handle throwables that are not
exceptions and these end up silently lost. This commit adds a check
after the runnable returns that inspects the status of the future. If an
unhandled throwable occurred during execution, this throwable is
propogated out where it will land in the uncaught exception handler.

* master:
  Backported synced-flush PR to v5.6.8 and v6.2.2
  Move more XContent.createParser calls to non-deprecated version (elastic#28672)
  Move more XContent.createParser calls to non-deprecated version (elastic#28670)
  Build: Group archive and package distribution projects (elastic#28673)
  [DOCS] Add supported token filters
  [TEST] bump timeout in testFetchShardsSkipUnavailable to 5s
  Relax remote check for bwc project checkouts (elastic#28666)
  [TEST] Synchronize searcher list in IndexShardTests
  [TEST] packaging: function to collect debug info (elastic#28608)
  Compute declared versions in a static block
  Docs: Remove references to elasticsearch directory in plugins (elastic#28647)
  Remove snapshot conditional for bwc snapshots (elastic#28657)
  Removed redundant JSON object from Put Mapping docs (elastic#28514)
  Update threadpool.asciidoc target_response_time (elastic#28655)

* master:
  Add a note to the docs that _cat api `help` option cannot be used if an optional url param is used (elastic#28686)
  Lift error finding utility to exceptions helpers
  Change "tweet" type to "_doc" (elastic#28690)
  [Docs] Add missing word in nested.asciidoc (elastic#28507)
  Simplify the Translog constructor by always expecting an existing translog (elastic#28676)
  Upgrade t-digest to 3.2 (elastic#28295) (elastic#28305)
  Add comment explaining lazy declared versions