Handle non-existant categories and urls

[ticua07]

Handle urls that don't exist:

This happens when accessing https://svgl.app/doesnotexist and because there isn't an +error.svelte present, a default message is shown instead.

Also, there isn't any validation that the category exists, so when changing the url to an invalid category no logos will appear

[vercel]

Someone is attempting to deploy a commit to a Personal Account owned by @pheralb on Vercel.

@pheralb first needs to authorize it.

[ticua07]

This is my first pull request ever, any suggestions are appreciated!

[pheralb]

Thanks for contributing 🥹🚀