Add error handling to extension load

[nwmac]

Summary

Fixes #13205

Technical notes summary

This PR improves the error handling when we load extensions.

I checked out the elemental-ui extension with the 2.9 branch, built it and served it up locally and developer loaded it into a 2.11 UI running locally - to reproduce trying to load a Vue 2 extension into Vue 3.

We don't have any exception handling in the load - the script for the extension loads, so our onloadhandler gets called. The script has an exception when it is evaluated at load time (error reading 'extend'), but this does not affect our code path - we check window[id]` is set, but because the injected script errored, it is an empty object and does not have the default function, so the real error for us is when we call that method here - https://github.com/rancher/dashboard/blob/master/shell/core/plugins.js#L116 - this throws an unhandled exception.

In the PR, I have added a check for the default function and added a try/catch around the call of it.

With this, the UI handles the load error. Note when running in dev, you will get the red overlay saying there is an unhandled exception (this is due to the error with the script load), but if you close it, our UI is working fine - before, you'd be stuck with the loading spinner. In production you wont' get the red overlay.

Areas or cases that should be tested

Extension loading

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