fix(icon): add better warning when loading icons

[liamdebeasi]

In ionic-team/ionic-framework#28445 we received feedback that accidentally referring to an icon by name without first registering it is confusing because devs get a cryptic "invalid URL" error. This PR attempts to improve that experience by providing a more helpful message and giving developers steps they can take to fix the issue.

main	branch

One note: This warning will be logged every time the icons tries to load. We currently load the icon in connectedCallback and componentDidLoad, so this warning can be logged twice on page load. In Angular, one more warning may be logged if the icon is in a router outlet (since the elements are moved into ion-router-outlet on load which means they are first removed from the DOM, so connectedCallback fires twice.). We could add a cache that tracks if we already warned about each icon per element instance if reviewers prefer (though that will slightly increase the complexity of this fix).

[averyjohnston]

I'm okay with the fix as-is; I would rather the warning log too many times than open the risk of a bug in the cache causing it to not log when it needs to.