Hbal.hs: rename functions that deal with early termination

In Hbal.hs, it's easy to get lost in the flow of exec* functions
because their names are similar and don't convey their different
purposes (e.g. runJobSet, execJobSet, execWrapper).

This patch renames 'runJobSet' to 'execWithCancel', and 'execWrapper'
to 'execCancelWrapper', since these two functions deal, in particular,
with early termination when the user presses ^C.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>