add exception handling to remove_keymap

Created by: zeffii

def remove_keymap():

    for km, kmi in nodeview_keymaps:
        try:
            km.keymap_items.remove(kmi)
        except Exception as e:
            err = repr(e)
            if "cannot be removed from 'Node Editor'" in err:
                print('keymaps for Node Editor already removed by another add-on, sverchok will skip this step in unregister')
                break

    nodeview_keymaps.clear()

or the more crude " hair on fire "

def remove_keymap():

    for km, kmi in nodeview_keymaps:
        if kmi.name in km.keymap_items:
            km.keymap_items.remove(kmi)

    nodeview_keymaps.clear()