RPCError is incorrectly re-encoded as kServerError in vmService

[jyameo]

The _routeRequest method in vm_service.dart has a bug where it catches and re-encodes RPCError exceptions as kServerError, losing the original error code and information.

This causes issues when code specifically throws an RPCError with a particular error code (e.g., kIsolateCannotReload), as that error code is lost and replaced with kServerError.

Expected Behavior
When an RPCError is thrown, it should be preserved as-is without re-encoding. Only non-RPCError exceptions should be wrapped as kServerError.