fix: improve error message for migration errors when slot would be renamed

[lfrancke]

The warning would just say

This migration would change the name of a slot making the component unusable

Now it says

This migration would change the name of a slot (derived to derived_1) making the component unusable

I'm totally open to changing the message itself.
For me, as a newbie, it was really unhelpful to not have any context so I included the name of the "bad" slot here.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 895b9a0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[paoloricciuti]

Looks good, thanks! 🧡

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15841