basic error message from rniEval

[reactormonk]

Very basic. How would you prefer the integration with the current structure? I can't create an REngineEvalException from the JNI because I don't have access to the engine.

Long version with REngineEvalException:

  jclass clazz = (*env)->FindClass(env, "org/rosuda/REngine/REngineEvalException");
  jstring message = (*env)->NewStringUTF(env, R_curErrorBuf())
  jmethodID init = (*env)->GetMethodID(env, cls, "<init>", "(Lorg/rosuda/REngine;Ljava/lang/String;)V");
  jobject exception = (*env)->NewObject(env, clazz, init, engine, message)
  (*env)->Throw(env, exception);

[reactormonk]

Anyone got time to review this?