lisa.wlgen.workload: Propagate exception from _run()'s exit path
lisa.wlgen.workload: Propagate exception from _run()'s exit path

Re-raise immediately the exceptions of Workload._run() from within
Workload.__exit__(), instead of just raising upon access to the "output"
attribute of the context manager.

credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa