Update xlib.py
`MissingFunctionException` from `pyglet/gl/lib.py` (inherits`Exception`) does not have a `.message` attribute. Not sure if that's part of `BaseException`, but this caused runtime errors on xlib machines.
The specific error being:
```
AttributeError: 'MissingFunctionException' object has no attribute 'message'
```
@@ -247,7 +247,7 @@ class BaseXlibContext(Context):
elif self._have_SGI_swap_control:
glxext_arb.glXSwapIntervalSGI(interval)
except lib.MissingFunctionException as e:
- warnings.warn(e.message)
+ warnings.warn(e)
def get_vsync(self):
return self._vsync
