Worker crash in-between running testitems should error gracefully

[nickrobinson251]

A worker crashing in between running testitems currently:

• causes an exception to be thrown, preventing outputting the test report
• the worker’s failure won't be logged until the end because of the (@sync/@spawn pattern here).

In theory, nothing should be running on the worker between test-items, but that relies on intended usage. It is possible for a testitem to start a background task (e.g. start a server) and forget to shut it down at the end of the testitem, and for that task to then (accidentally) bork the worker in-between test-items. And in this case the error should make clear what happened.

Proposed improvements:

• A test failure in between tests should be caught, and reported at the end
  • We should also restart the worker in this case
• We should still print the standard test failures & successes report
• We should still throw an exception

Proposed output format:

An error caused Worker 23423 to crash in between tests. More details after the test report, below.
Test Summary:                                      | Pass  Error  Total      Time
RAICode                                            |    ...
   ...

EXCEPTION CAUGHT WHEN SENDING TESTITEM TO WORKER:
Worker 23423 crashed when attempting to send testitem "..."
Previous 10 tests run on this worker:
- ...
- ...

<throw the exception>
Stacktrace:
...