[FEATURE] More informative indexing errors (for category indexing)

[andrzejnovak]

Consider:

w = 1e-2  
x = np.random.normal(loc=0.5, scale=0.2, size=1000)
axes = 'ABCDEFG'
h = hist.Hist(
          *[hist.axis.StrCategory([], name=name, growth=True) for name in axes],
          hist.axis.Regular(3, 0, 1, name="x", label="axis"),   
    )
h.fill(**{k :np.random.choice(list("ABCDEFG"), 1000) for k in axes}, **{"x":x})

h[{"A" : "X"}]

Yields:

IndexError: index 7 is out of bounds for axis 0 with size 7

If the IndexError showed the unconverted value as well as name of the axis it would be great

[henryiii]

Related to scikit-hep/boost-histogram#387