See #2072

Shows the property that doesn't support it.
Need to still update the tests though.

For example, `fig.update_layout(width_x=1)` gives the following error
message:

TypeError: 'NoneType' object is not subscriptable

Invalid value received for the 'width' property of layout

    The 'width' property is a number and may be specified as:
      - An int or float in the interval [10, inf]

Property does not support subscripting:
width_x
~~~~~

because `fig.layout['width']` can't be subscripted (you can't do
`fig.layout['width']['x'] = 1`)

But overrode the __str__ method.

It used to raise ValueError but PlotlyKeyError subclasses KeyError and
is the more relevant exception. PlotlyKeyError maintains the nice
printing of the errors, unlike KeyError.

Because lookup in subclasses of BasePlotlyType and BaseFigure should
throw KeyError.

I believe the new behaviour is consistent with the previous exception
behaviour, but we will run the CI tests to be sure.

Before the subscripting error was only thrown for NoneType objects, but
because stuff like `"hello"["hi"]` and `1["hey"]` also are invalid, and
these also throw TypeError, then these throw the subscripting error
message too. HOWEVER, this error is casted to ValueError on property
assignment and PlotlyKeyError on property read, to keep consistency
among the exception types.

and underneath the whole offending part of the path e.g.,

the_badpart_of_the_path
    ^^^^^^^

for both property and subscripting errors.

then if that fails, try and print the int, then if that fails, truely
fail. This allows taking the length of unicode objects in Python2.

need to add tests

Tests in tests/test_core/test_errors/test_dict_path_errors.py
not yet updated though.

Also tripped when no underscore paths are used, e.g.,

fig.data[0].line["colr"] = "blue"