QGIS: don't crash on unknown layers in database

[visr]

Currently both Ribasim Python and the core silently ignore extra unknown tables. This makes QGIS do the same.

An error has occurred while executing Python code: 

KeyError: 'Basin / boundaryconcentration' 
Traceback (most recent call last):
  File "C:\Users/visser_mn/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\ribasim_qgis\widgets\dataset_widget.py", line 286, in open_model
    self.load_geopackage()
  File "C:\Users/visser_mn/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\ribasim_qgis\widgets\dataset_widget.py", line 221, in load_geopackage
    nodes = load_nodes_from_geopackage(geo_path)
  File "C:\Users/visser_mn/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\ribasim_qgis\core\nodes.py", line 870, in load_nodes_from_geopackage
    nodes[layername] = NODES[layername](path)
KeyError: 'Basin / boundaryconcentration'