AWY validation doesn't catch missing demand table when doing valuation

[phargogh]

Nfamara recently contacted me about an exception he was experiencing (see below). This was caused because he had provided a valuation table, but not provided a demand table. This dependency is not clearly identified in the UG, nor is it clearly identified in validation.

ERROR:taskgraph.Task:Something went wrong when adding task create_ws_id_vector_output (13), terminating taskgraph.
Traceback (most recent call last):
  File "/Users/jdouglass/Downloads/2025-01-31-nfamara-awy/env312/lib/python3.12/site-packages/taskgraph/Task.py", line 674, in add_task
    new_task._call()
  File "/Users/jdouglass/Downloads/2025-01-31-nfamara-awy/env312/lib/python3.12/site-packages/taskgraph/Task.py", line 1093, in _call
    payload = self._func(*self._args, **self._kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jdouglass/Downloads/2025-01-31-nfamara-awy/env312/lib/python3.12/site-packages/natcap/invest/annual_water_yield.py", line 945, in write_output_vector_attributes
    compute_watershed_valuation(target_vector_path, valuation_df)
  File "/Users/jdouglass/Downloads/2025-01-31-nfamara-awy/env312/lib/python3.12/site-packages/natcap/invest/annual_water_yield.py", line 1118, in compute_watershed_valuation
    rsupply_vl = ws_feat.GetField('rsupply_vl')
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jdouglass/Downloads/2025-01-31-nfamara-awy/env312/lib/python3.12/site-packages/osgeo/ogr.py", line 4405, in GetField
    raise KeyError("Illegal field requested in GetField()")
KeyError: 'Illegal field requested in GetField()'