Use all available processors.

Summary:
Better logging on errors. Use fmt.Fprintf for output to stdout and stderr
when calling through from command line.

Test Plan:
none necessary; How would we test we're actually using max number
of processors? Anyone have a good idea, or not worth the effort?

Reviewers: Tobias, bdarnell, andybons

Reviewed By: andybons

Differential Revision: http://phabricator.cockroachdb.org/D71