[Flutter GPU] Propagate native error detail back through the Dart API #186768

Use case

Native failures in Flutter GPU currently surface to Dart only as a bool returned from the Internal* bindings. Dart re-raises a generic Exception("Failed to append draw") (and similar). Impeller emits failure detail via VALIDATION_LOG to stderr, but none of that detail makes it back to the Dart caller. Diagnosing failures from Dart is harder than it needs to be, especially as more callers adopt the API.

Proposal

Plumb Impeller's fml::Status (or an equivalent structured error type) from the native side back through Flutter GPU's Internal* bindings, and surface it in Dart as a descriptive exception that names the operation and includes the underlying status message. Apply this consistently across the Flutter GPU Dart API rather than fixing it ad hoc per method.

Discussed in #186639.