Robustly handle missing icons.

[rtibbles]

Description

• This makes the KIcon component properly handle when it is passed an icon name that does not exist, preventing TypeErrors
• Defaults to a brokenImage icon in the cases that the icon is not recognised

Issue addressed

Fixes #390

Before/after screenshots

Added documentation of this behaviour to the main KIcon docs.

(optional) Implementation notes

At a high level, how did you implement this?

Does this introduce any tech-debt items?

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical and brittle code paths are covered by unit tests
• The change has been added to the changelog

Reviewer guidance

• Is the code clean and well-commented?
• Are there tests for this change?
• Are all UI components LTR and RTL compliant (if applicable)?
• Add other things to check for here

Comments

[marcellamaki]

This looks good to me. I like the approach. And thanks, @indirectlylit for the notes on the docs!