Windows setup: route the stale-manifest failure through Exit-SetupFailure (#7569)

The manifest-removal guard added in #7492 exits with a bare 'exit 1', so in
Tauri mode the installer never emits the [TAURI:ERROR] line and the desktop
UI falls back to a generic failure instead of naming the cause. Every other
failure path in studio/setup.ps1 goes through Exit-SetupFailure, and
tests/sh/test_tauri_retry_failure_context.sh asserts that invariant, so
'Repo tests (CPU)' has been red on main since that merge.

Co-authored-by: danielhanchen <unslothai@gmail.com>

studio/setup.ps1

@@ -3122,7 +3122,7 @@ sys.exit(0 if install_manifest.remove_manifest() else 1)
 if (-not $_ManifestDropped) {
     Write-Host "[ERROR] Could not remove the stale unsloth_install_manifest.json." -ForegroundColor Red
     Write-Host "        Refusing to install behind a marker that still reports this venv as complete." -ForegroundColor Red
-    exit 1
+    Exit-SetupFailure "Could not remove the stale unsloth_install_manifest.json"
 }
 
 if ($script:UnslothVerbose) {