fix(composable): disable caching & return error object

[smarroufin]

Description

I've had two issues using useNewsletterSubscribe.

1. There is cache on the request so re-triggering a request with the same email only returns cache.
2. I am not able to explore the error object, so I can determine if the error is about the user being already subscribed or anything else.

Fix

1. Use $fetch instead of useFetch to always trigger the request without using cache.
2. Catch the error exception and return the error object.

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[Baroshem]

Nice work @smarroufin !

I am happy to merge it :)