Prevent Qt4 from stopping the interpreter

Hi,

This commit prevents Qt4 from stopping the interpreter:
If there's no X server available, or if you export DISPLAY="", and try to initialize a QApp, Qt4 stops:
: cannot connect to X server

It's a non-feature of Qt4 to not use exceptions to be lightweight.
But in python, that's not really the spirit, we'd expect a python backtrace, so here's what the patch does:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/schueller/projects/matplotlib/install/lib/python2.7/site-packages/matplotlib-1.3.x-py2.7-linux-x86_64.egg/matplotlib/pyplot.py", line 348, in figure
    **kwargs)
 ...
    raise RuntimeError( 'Qt4 failed to initialize ' + p.stderr.readline().rstrip() )     
RuntimeError: Qt4 failed to initialize : cannot connect to X server

And it's more like the other backends do:
_tkinter.TclError: couldn't connect to display ""


Regards.