Raise informative error when `get_recharge()` yields an empty result

[dbrakenhoff]

Currently the code below raises a KeyError, I believe it would be more informative to catch this error and raise a more informative error on the NLmod side, e.g. warn about an empty ObsCollection ( maybe with the suggestion to check if recharge is available for this period)?

import nlmod
import pandas as pd

ds = nlmod.get_ds([100000, 110000, 420000, 430000])
time = [pd.Timestamp.now().normalize()]
ds = nlmod.time.set_ds_time(ds, start=time[0] - pd.Timedelta(days=21), time=time)
knmi = nlmod.read.knmi.get_recharge(ds)

The test tests/test_005_external_data.py::test_get_recharge_not_available will have to be modified to reflect that change.

[rubencalje]

Now I get a ValueError: ValueError: no data available for time series'RD-EV24' on date 2025-10-29 00:00:00, which I think is a useful error.

In PR #517 I changed the error-type (from KeyError to ValueError) in the test.

I think this closes the issue?