Hbal.hs: use Result (), not Bool, as return value for exec*

Previously, functions in Hbal.hs related to execution of jobsets were
returning only IO Bool, and printing any errors they found directly to
stderr on their own.

I'm going to be moving some of these functions to a library module in
future commits, and it makes sense that they won't print to stderr, but
rather return an error condition. To make diffs more readable, I change the
return value in Hbal.hs itself, so that the next commit deals only with the
move. It's now `main` that prints any Bad result to stderr.

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