Allow nesting BaseHandler exception handler decorators.
Previously, when multiple decorated methods were nested and
one of them raised an exception, the exception has been handled
by all the decorators.
This is not right way, because exception which should be handled
just by the `fail_artifact_build_on_handler_exception` decorator
was later re-raised and handled also by the `fail_event_on_handler_exception`
making the `fail_artifact_build_on_handler_exception` useless in this
combination of decorators.
This commit fixes this by changing the decorators to handle the
exception only once. That means only the first decorator will
handle the exception.