early reporting of errors to users on the CLI

[dawuud]

Perhaps: we should replace the tahoe start command something very similar that tails the long looking for a magic string and then exits. If magic completion string is not found then report an error to the user telling them to look at the log file for more info.

We would like to systematically iterate towards an asynchronous startup for tahoe... because we've got an accounting database that needs async startup... and the accounting db is needed for everything else. There are many failure paths that can occur on startup and we'd like to catch all of them as soon as possible and report them to the user instead of what tahoe start does: background the daemon and report nothing to the CLI. Silently fail.