MB-44253: Remove try/catch from Executor*::runTask methods

[jimwwalker]

Slightly adapted Dave Rigby's patch:

commit 3d4519d
Author: Dave Rigby daver@couchbase.com
Date: Thu Feb 11 14:01:47 2021 +0000

MB-44253: Remove try/catch from Executor*::runTask methods

These try/catch blocks prevent unhandled exceptions in GlobalTasks
from triggering std::terminate, and in turn generating a Breakpad
minidump with the state of the process when the exception was thrown
(as opposed to where it was caught).

Started a discussion with Folly about a more generic way to address
this (i.e. making the try/catch somehow optional) - see
https://github.com/facebook/folly/issues/1525 - but in the short-term
simply remove the invokeCatchingExns() wrapper from our branch.

(Note: Currently we only use ThreadPoolExecutor and hence we only
/need/ to change that one, but for consistency / possible future use
change all instances).