- Added "logical tracebacks" that show functionally what the buildout
  was doing when an error occurs.  Don't show a Python traceback
  unless the -D option is used.

- Added a -D option that causes the buildout to print a traceback and
  start the pdb post-mortem debugger when an error occurs.


git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@73082 62d5b8a3-27da-0310-9561-8e5933582275