[Piglit] piglit-summary.py exception on file not found

[Brian Paul]

Hi Dylan,

Could we have piglit-summary.py print a simple error message when the 
file is not found, rather than raising an exception:

$ ./piglit-summary.py -s  foo
/home/brian/projects/piglit-mingw32-new/piglit/framework/test/base.py:77: UserWarning: 
Timeouts are not available
   warnings.warn('Timeouts are not available')
Traceback (most recent call last):
   File "./piglit-summary.py", line 35, in <module>
     console([i.decode('utf-8') for i in sys.argv[1:]])
   File 
"/home/brian/projects/piglit-mingw32-new/piglit/framework/exceptions.py", line 
51, in _inner
     func(*args, **kwargs)
   File 
"/home/brian/projects/piglit-mingw32-new/piglit/framework/programs/summary.py", 
line 169, in console
     summary.console(args.results, args.mode or 'all')
   File 
"/home/brian/projects/piglit-mingw32-new/piglit/framework/summary/console_.py", 
line 103, in console
     results = Results([backends.load(r) for r in results])
   File 
"/home/brian/projects/piglit-mingw32-new/piglit/framework/backends/__init__.py", 
line 178, in load
     'No module supports file extensions "{}"'.format(extension))
framework.backends.BackendError: No module supports file extensions ""


I had a typo in my filename and it took me a while to realize my 
mistake.  I think a regular error message would be nicer.

-Brian