Fix strange AttributeError if libxcb-image.so is not available

Also fixes it for all other libraries which are imported via ctypes. A proper error message is now printed on stderr, internally the modules requiring these libraries fail to import with ImportError.

for future reference, this is the error message from before:

    Traceback (most recent call last):
      File "../scripts-3.3/pyxtrlock", line 15, in <module>
        import pyxtrlock.xcb as xcb
      File "/home/horazont/Projects/python/pyxtrlock/build/lib/pyxtrlock/xcb.py", line 497, in <module>
        libxcb_image.xcb_create_pixmap_from_bitmap_data
      File "/usr/lib64/python3.3/ctypes/__init__.py", line 361, in __getattr__
        func = self.__getitem__(name)
      File "/usr/lib64/python3.3/ctypes/__init__.py", line 366, in __getitem__
        func = self._FuncPtr((name_or_ordinal, self))
    AttributeError: /usr/bin/python3: undefined symbol: xcb_create_pixmap_from_bitmap_data