This patch is similar to that proposed by Jeremy. The proposed patch altered
the interface of TestResult such that it would be passed the error
information as a string rather than an exc_info() tuple.

The implemented change leaves the interface untouched so that TestResults
are still passed the tracebacks, but stor them in stringified form for
later reporting.

Notes:
- Custom subclasses of TestResult written by users should be unaffected.
- The existing 'unittestgui.py' will still work with this module after the
  change.
- Support can later be added to pop into the debugger when an error occurs;
  this support should be added to a TestRunner rather than to TestCase itself,
  which this change will enable.

(Jeremy, Fred, Guido: Thanks for all the feedback)