fix UnitOperationError exception when the exponent is unyt_array or u…

[l-johnston]

…nyt_quantity type

Raising a value to a unyt_array type causes an AttributeError in the str() of UnitOperationError since unit2 is a unyt_array and not a Unit.

>>> from unyt import m
>>> 10**(1*m)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/mnt/d/Python/unyt/unyt/unyt/array.py", line 1669, in __array_ufunc__
    raise UnitOperationError(ufunc, u0, u1)
unyt.exceptions.UnitOperationError: <exception str() failed>

[ngoldbaum]

Good catch on the bug!

Assuming my reading of the pytest docs is correct, I'm pretty sure this can be phrased like this:

with pytest.raises(UnitOperationError) as excinfo:
    assert type(excinfo.value.unit1) is Unit
    assert type(excinfo.value.unit2) is Unit

However I think the way you have it is actually simpler so I'm going to merge it as-is.