Made more fault tolerant and gave more explicit error message.

If autopep8 was not properly installed, Spyder would fail to load the Preferences widget altogether, because the AutoPEP8ConfigPage was trying to access FIX_LIST and DEFAULT_IGNORE, which were not defined.

Now, a message is shown telling the user which versions of autopep8 and pep8 are required (I posted an issue on autopep8 letting them know about the implicit dependence on pep8 1.4.2).