GDP find Error mis handles non-regular expressions

[Curtis Hovey]

This is most like cause by the refactoring to a threaded finder. Something in the hand-off is missing. Consider reverting te threaded work because gdp plugins cannot take advantage of threads.

Traceback (most recent call last):
  File "/home/curtis/.gnome2/gedit/plugins/gdp/find.py", line 270, in on_find_in_files
    find_worker.run()
  File "/home/curtis/.gnome2/gedit/plugins/gdp/find.py", line 105, in run
    self.find_params.pattern, substitution=self.substitution):
  File "/home/curtis/.gnome2/gedit/plugins/gdp/find.py", line 36, in find_matches
    match_re = re.compile(match_pattern)
  File "/usr/lib/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python2.7/re.py", line 245, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat