[6.x] Handle cancelled requests when generating slugs

[duncanmcclean]

This pull request fixes an issue where the slug field would stop updating when typing on slow network connections.

This was happening because when a slug request was cancelled (due to the user continuing to type), the cancelled request's promise was resolving with undefined. This undefined value would then propagate through to Slugify.vue, setting this.slug = undefined. When the to watcher detected this mismatch between the parent's slug value and this.slug, it would set shouldSlugify = false, permanently disabling slug generation until the page is reloaded.

This PR fixes it by allowing cancelled requests to throw their error (instead of returning undefined) and ignoring CanceledError exceptions in Slugify.vue so cancelled requests don't interfere with the slug field state.

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Fixes #14206