GLdispatch: Return an error when glGetError is called without a context

When there isn't a current context, libglvnd will (try to) plug in a dispatch table that sends everything to no-op stubs which do nothing and return zero. That's fine for most things, but glGetError ought to return a non-zero value, since zero is GL_NO_ERROR.

This adds a special case to gen_gldispatch_mapi.py so that glGetError will return GL_INVALID_OPERATION.