Describe the bug

When InstanceStart returns an error (e.g. container does not exist), both strategies fail to handle it properly:

• Dynamic: The loading page shows an empty instance with no name and no status instead of displaying the error. The page keeps auto-refreshing forever because the session is never stored and InstanceStart is retried on every request.
• Blocking: RequestReadySession polls RequestSession until timeout, retrying InstanceStart on every tick. The individual instance error is embedded in InstanceInfoWithError.Error but never checked — only IsReady() is evaluated, which always returns false.

Context

• Sablier version: 1.11.1
• Provider: all (confirmed with Podman and Docker)
• Reverse proxy: N/A (direct API access)
• Sablier running inside a container? No

Expected behavior

• Dynamic: The loading page should display the instance name and the error message so the user can understand what went wrong. The theme already supports error rendering via {{ if $instance.Error }}.
• Blocking: The endpoint should return the error immediately instead of retrying until timeout.

Additional context
Root cause: InstanceInfoWithError.Error is set when InstanceStart fails, but neither the dynamic rendering path nor the blocking polling loop checks it.