Move the function taking care of closing the Browser Console to shared-head so
we can close it before closing the tabs opened during the test.

While attaching the worker, check that the Worker Debugger isn't closed, and
if it is, reject the promise. On the target list, catch rejection while attaching
and simply bail out so we don't call the rest of the code in onTargetAvailable.

Those guards are not enough to prevent any failure while attaching, so we're
wrapping calls to `attachAndInitThread`/`_onThreadInitialized` in try/catch
blocks to avoid test failures.

Differential Revision: https://phabricator.services.mozilla.com/D88766