Add a little additional logging to pub's version solver.

This helped me track down issue 16910, and it seemed general enough to
keep.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//172873006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@32838 260f80e4-7a28-3924-810f-c04153c831b5