Add a suggestion for how to resolve chunk manager not available error

[jsignell]

Just tries to add a suggestion about which library to install. @keewis had done something similar for guess_chunkmanager already.

For instance, if you have cubed but not cubed-xarray:

import cubed
import cubed.random

import xarray as xr

t_length = 50
spec = cubed.Spec(allowed_mem="2GB")

u = cubed.random.random((t_length, 1, 987, 1920), chunks=(10, 1, -1, -1), spec=spec)
v = cubed.random.random((t_length, 1, 987, 1920), chunks=(10, 1, -1, -1), spec=spec)

ds = xr.Dataset(
    dict(
        anom_u=(["time", "face", "j", "i"], u),
        anom_v=(["time", "face", "j", "i"], v),
    )
)
result = ds**2
result.compute()