Chore/error handling improvements

[aaron-goff]

Related Issue

N/A

Description

We use several methods that pre-suppose the existence of an element. In cases where these elements do not exist (or can't be found), we receive NoSuchElementException. These failures can be a little difficult to troubleshoot, so I've added more robust exception handling as well as unit tests.

Definitely open to feedback about better exception error text, or ways to implement the exception unit tests.

Checklist

• [x] Have you added an explanation of what your changes do?
• [x] Have you written new tests for your core changes, as applicable?
• [x] Docs have been added / updated (for bug fixes / features)

[aaron-goff]

@mrk-han Do you want me to bump this to 0.19.4?