Gracefully handle invalid icon selection

[sentry-io]

If an unregistered icon name is passed to KIcon, then it will cause TypeErrors from trying to access properties of undefined.

Sentry Issue: STUDIO-FCT

TypeError: Cannot read properties of undefined (reading 'icon')
  at lib_KIconvue_type_script_lang_js_.computed.svgIconComponent (../../../../node_modules/kolibri-design-system/lib/KIcon/index.vue:68:1)
  at Watcher.prototype.get (../../../../node_modules/vue/dist/vue.runtime.esm.js:4495:25)
  at Watcher.prototype.evaluate (../../../../node_modules/vue/dist/vue.runtime.esm.js:4597:21)
  at svgIconComponent (../../../../node_modules/vue/dist/vue.runtime.esm.js:4851:17)
  at KIconvue_type_template_id_36cbad00_scoped_true_render (../../../../node_modules/kolibri-design-system/lib/KIcon/index.vue?5e75:5:17)
...
(45 additional frame(s) were not displayed)

[sentry-io]

Sentry issue: STUDIO-FJV

[rtibbles]

Fixed in #391