## Motivation

The error-chain formatting (`error: <msg>` + `* <source>` lines,
bold-red prefix) used to live in vite-plus at the napi boundary
(voidzero-dev/vite-plus#784). That meant any tweak to error output — or
any test that asserts on it — required building the whole vite-plus napi
binding and running through the JS CLI.

Moving the formatter into vite-task lets us iterate on and snapshot-test
the exact user-visible error output directly with `cargo test`, no
vite-plus build needed. vite-plus can then drop its napi-side formatter
since `Session::main` no longer surfaces errors across the boundary — it
prints them itself and returns only an exit status.

## Changes

- New `vite_task::print_error(&anyhow::Error)` — writes `error:
<top-level>` (bold red on TTY) plus `\n* <source>` for each link in the
chain, mirroring the format vite-plus#784 introduced.
- `Session::main` signature: `anyhow::Result<ExitStatus>` →
`ExitStatus`. It prints errors itself via `print_error` and returns
`FAILURE`; the caller just exits with the status.
- `vt` binary uses `print_error` for `Session::init` errors too, so any
error escaping the CLI uses the same format.
- e2e snapshots updated for the new format (`Error:` → `error:`,
anyhow's `Caused by:\n ...` block → `* ...` lines).

vite-plus is pinned to a vite-task revision and will be updated
separately to drop its napi-layer formatter.

https://claude.ai/code/session_012BLuM9bhiDFBfc8U1CMst8

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Co-authored-by: Claude <noreply@anthropic.com>