Changed TestResult to store only the text representation of an error.

Changed TestResult to store only the text representation of an error.

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)