For templates with `change_mode = "script"`, we set a driver handle in the
poststart method, so the template runner can execute the script inside the
task. But when the client is restarted and the template contents change during
that window, we trigger a change_mode in the prestart method. In that case, the
hook will not have the handle and so returns an errror trying to run the change
mode.
We restore the driver handle before we call any prestart hooks, so we can pass
that handle in the constructor whenever it's available. In the normal task start
case the handle will be empty but also won't be called.
The error messages are also misleading, as there's no capabilities check
happening here. Update the error messages to match.
Fixes: https://github.com/hashicorp/nomad/issues/15851
Ref: https://hashicorp.atlassian.net/browse/NET-9338
c280891703