More descriptive error when accessing an unhandled mobject attribute

[friedkeenan]

Motivation

Because #995 added a __getattr__ method to Mobject, all incorrect accesses to a mobject's attributes goes through that method, but the resulting error does not have an accompanying message (sorry). This fixes that.

Overview / Explanation for Changes

Adds an error message to the error raised at the end of __getattr__. It uses f"{self} ..." to take advantage of mobjects' custom representations, particularly with Text and Tex and such which include their content in their representation.

Oneline Summary of Changes

- More descriptive error when accessing an unhandled mobject attribute (:pr:`1059`)

Testing Status

This change shouldn't affect any tests.

Acknowledgements

• I have read the Contributing Guidelines

Reviewer Checklist

• Newly added functions/classes are either private or have a docstring
• Newly added functions/classes have tests added and (optional) examples in the docs
• Newly added documentation builds, looks correctly formatted, and adds no additional build warnings
• The oneline summary has been included in the wiki

[huguesdevimeux]

Tests are failing for 3.8

And, do you mind adding a test for this?

[friedkeenan]

Welp, tests should pass now, though now we don't take advantage of mobject representations. My bad for not testing what I thought would be an utterly simple change.

Also changed a test that was explicitly checking for an attribute error for a missing attribute so that it looks for the message too.