Malformed exception does not include variable information

[BorjaEst]

I have the following dataset:

<xarray.Dataset>
Dimensions:      (... time: 24)
Coordinates:
  ...
  * time         (time) datetime64[ns] 1960-01-16T12:00:00 ... 1961-12-16T12:...
Data variables:
    ...
    zmo3         (time, lev, lat, lon) float32 dask.array<chunksize=(24, 31, 22, 1), meta=np.ndarray>
Attributes:
    ...
    Conventions:  CF-1.4

Where the attr cell_measures is malformed:

dataset.cf.axes
Traceback (most recent call last):
  ...
ValueError: attrs['cell_measures'] = 'N/A' is malformed.

I know the malformed attr is inside the zmo3 variable, but the exception does not provide that info and makes handling the execption quite dificult:

    try:
        dataset.cf[variable_standard_name]
    except ValueError as err:
        # err => ValueError("attrs['cell_measures'] = 'N/A' is malformed.")
        pass  # How to del the malformed attr if I do not know the variable

It would be nice that the Value error includes the variables that raised the issue.

[dcherian]

With #350, there should be a warning instead of an error and it should print the variable name.