Raise pint.DimensionalityError instead of ValueError

If you try to convert a pint.Quantity to an incompatible unit you get a pint.DimensionalityError, whereas if you try to convert a quantified DataArray to an incompatible unit you get a ValueError. These should give the same type of error.

In [1]: import pint_xarray

In [2]: import xarray as xr

In [3]: import pint

In [4]: mass_q = pint.Quantity(10, units='kg')

In [5]: mass_q.to('seconds')
---------------------------------------------------------------------------
DimensionalityError                       Traceback (most recent call last)
<ipython-input-5-86c449341523> in <module>
----> 1 mass_q.to('seconds')

~/Documents/Work/Code/pint/pint/quantity.py in to(self, other, *contexts, **ctx_kwargs)
    722         other = to_units_container(other, self._REGISTRY)
    723 
--> 724         magnitude = self._convert_magnitude_not_inplace(other, *contexts, **ctx_kwargs)
    725 
    726         return self.__class__(magnitude, other)

~/Documents/Work/Code/pint/pint/quantity.py in _convert_magnitude_not_inplace(self, other, *contexts, **ctx_kwargs)
    671                 return self._REGISTRY.convert(self._magnitude, self._units, other)
    672 
--> 673         return self._REGISTRY.convert(self._magnitude, self._units, other)
    674 
    675     def _convert_magnitude(self, other, *contexts, **ctx_kwargs):

~/Documents/Work/Code/pint/pint/registry.py in convert(self, value, src, dst, inplace)
   1001             return value
   1002 
-> 1003         return self._convert(value, src, dst, inplace)
   1004 
   1005     def _convert(self, value, src, dst, inplace=False, check_dimensionality=True):

~/Documents/Work/Code/pint/pint/registry.py in _convert(self, value, src, dst, inplace)
   1915                 value, src = src._magnitude, src._units
   1916 
-> 1917         return super()._convert(value, src, dst, inplace)
   1918 
   1919     def _get_compatible_units(self, input_units, group_or_system):

~/Documents/Work/Code/pint/pint/registry.py in _convert(self, value, src, dst, inplace)
   1516 
   1517         if not (src_offset_unit or dst_offset_unit):
-> 1518             return super()._convert(value, src, dst, inplace)
   1519 
   1520         src_dim = self._get_dimensionality(src)

~/Documents/Work/Code/pint/pint/registry.py in _convert(self, value, src, dst, inplace, check_dimensionality)
   1034             # then the conversion cannot be performed.
   1035             if src_dim != dst_dim:
-> 1036                 raise DimensionalityError(src, dst, src_dim, dst_dim)
   1037 
   1038         # Here src and dst have only multiplicative units left. Thus we can

DimensionalityError: Cannot convert from 'kilogram' ([mass]) to 'second' ([time])

In [6]: mass_da = xr.DataArray(10).pint.quantify(units='kg')

In [7]: mass_da.pint.to('seconds')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/Documents/Work/Code/pint-xarray/pint_xarray/conversion.py in convert_units(obj, units)
    231     try:
--> 232         new_obj = call_on_dataset(
    233             convert_units_dataset, obj, name=temporary_name, units=units

~/Documents/Work/Code/pint-xarray/pint_xarray/compat.py in call_on_dataset(func, obj, name, *args, **kwargs)
     12 
---> 13         result = func(ds, *args, **kwargs)
     14 

~/Documents/Work/Code/pint-xarray/pint_xarray/conversion.py in convert_units_dataset(obj, units)
    216     if failed:
--> 217         raise ValueError(failed)
    218 

ValueError: {None: DimensionalityError()}

The above exception was the direct cause of the following exception:

ValueError                                Traceback (most recent call last)
<ipython-input-7-00816f2a573d> in <module>
----> 1 mass_da.pint.to('seconds')

~/Documents/Work/Code/pint-xarray/pint_xarray/accessors.py in to(self, units, **unit_kwargs)
    568         units = either_dict_or_kwargs(units, unit_kwargs, "to")
    569 
--> 570         return conversion.convert_units(self.da, units)
    571 
    572     def chunk(self, chunks, name_prefix="xarray-", token=None, lock=False):

~/Documents/Work/Code/pint-xarray/pint_xarray/conversion.py in convert_units(obj, units)
    238             failed[obj.name] = failed.pop(temporary_name)
    239 
--> 240         raise ValueError(format_error_message(failed, "convert")) from e
    241 
    242     return new_obj

ValueError: Cannot convert variables:
    incompatible units for variable None: Cannot convert from 'kilogram' ([mass]) to 'second' ([time])

Start in pint_xarray/conversion.py at convert_units and convert_units_dataset, where the traceback shows the conversion failure being wrapped, then follow the call from pint_xarray/accessors.py. Reproduce the incompatible kilogram-to-second conversion from the issue and verify that it raises pint.DimensionalityError rather than ValueError.