React in the UI after an exception in initialization

[jeffgca]

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We recently has an issue in WAT where loading of the app failed and this exception was raised:

Error: Could not decode CID: {"/":"bafkreifyss2n2jex7l5x2rkeyjnx4uhoatmxw2zvdndkioirru3vc2mina"}
    Immutable 25
    <anonymous> https://webnative-template.fission.app/:17
_layout.svelte-5b2610bc.js:1:1777
    Immutable 16
    <anonymous> https://webnative-template.fission.app/:17

It's quite possible that a developer could write some code in their app that also raises a similar catastrophic exception in app initialization. When the above error happened, the app UI got stuck on an infinite spinner. I would like to do the following instead:

1/ if debug=true, we change the UI to show the exception raised in gory detail.

2/ else we show a generic error route.