Use more specific `FetchError` for `useFetch` errors

Editor assessment

A well-specified patch. The subject states the change concretely — `useFetch` errors now surface as the more specific `FetchError` — with both API names backticked, making the entry precise and searchable. `fix(nuxt)` is reasonable: the prior behavior (generic errors) can fairly be characterized as a defect in error handling, though one could argue `feat` since typing improves; either maps to a compatible release here. Mood, casing, and punctuation are correct, and the header is short. What is missing is a body describing the practical benefit (stack traces, status codes, response access), which is exactly the why that would help users evaluate upgrading. Otherwise exemplary.