Fix bugs, add better logging.

Fix bugs, add better logging.

* dry_run semantics were improperly being triggered
  due to a logic bug.
* Created warnings.log and errors.log files, which
  are machine friendly lists of warning'd and error'd
  installs.
* Add the shorten number to make it easy for someone
  to grab a specific logfile.
* Add locking to migration; this means better error
  messages too
* Make call errors more expressive.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>