Make the sel error more descriptive when method is unset

[jsignell]

• Tests added

This came up in the tutorial and I was wondering if the error could be made a little clearer. Not sure if the error message should hint that a user might want to use method.

importnumpyasnpimportpandasaspdimportxarrayasxrarr=xr.DataArray(
data=np.arange(48).reshape(4, 2, 6),
dims=("u", "v", "time"),
coords={
"u": [-3.2, 2.1, 5.3, 6.5],
"v": [-1, 2.6],
"time": pd.date_range("2009-01-05", periods=6, freq="M"),
},
)
arr.sel(u=5, time="2009-04-28") # I removed `method="nearest"`

Before this PR:

...
KeyError: 5.0

After this PR:

...
KeyError: "not all values found in index 'u'. Did you mean to set `method=`?"

[dcherian]

Thanks @jsignell This is a nice improvement!

[mathause]

Thanks @jsignell!