I spent a fair amount of improving error handling along the RPC path,
as well making the code more consistent across the various files. Also
be more consistent in how the web interface invokes the backend and gets
errors back, specifically for errors that are generated when taking to a
remote cluster.

Add checks before every RPC to make sure the cluster is not disabled in
the database. Also check that we can actually reach the cluster, and
that the cluster is not offline (NoLogins()) before we try to do
anything. I might have to relax this a bit, but in general it takes a
couple of seconds to check, which is a small fraction of what most RPCs
take. Return precise errors for clusters that are not available, to the
web interface and show them to user.

Use webtasks more consistently between the web interface and backend
scripts. Watch specifically for scripts that exit abnormally (exit
before setting the exitcode in the webtask) which always means an
inter...