More flexibility when using {#await}

[alex0ide]

Is your feature request related to a problem? Please describe.
I'm trying to only catch errors on a promise, and to do so I'm forced to do {#await promise}{:catch error} (my code) {/await}

Describe the solution you'd like
Like the {#await promise then result} short-hand, I would love a {#await promise catch error} short-hand.

How important is this feature to you?
I'm trying to figure out a pattern for my Svelte routes that involves doing something like

const gotGroup = need('group', groupId) // this is a promise
$: group = $groups[groupId]

and then, in different parts of the code, I might want to catch an error, but not show anything while it's loading, and don't get the result either because I'm getting that from the store:

{#await gotGroup}{:catch error}
    {error}
{/await}

Additional context
Svelte compiler throws a warning because of an {#await} block being empty when doing {#await promise}{:catch error}

A question I have, separate from this feature and about the {#await} block, is if it's possible to use inside of class= or any other prop inside an element (class="{#await promise}loading{/await}"). If it's not, what's the alternative? I don't want to repeat code for the two paths.

[antony]

Duplicate of #3623

Please keep issues to a single request please - ignoring additional question