message: [cashlib] Throw in GetPubKey on invalid argument
  Previously the error would be logged, then segfault.
diff: @@ -840,7 +840,11 @@ extern "C" JNIEXPORT jbyteArray JNICALL Java_bitcoinunlimited_libbitcoincash_Pay
  if (len != 32)
  {
  - __android_log_print(ANDROID_LOG_VERBOSE, APPNAME, "secret has incorrect length\n");
  + std::stringstream err;
  + err << "GetPubKey: Incorrect length for argument 'secret'. "
  + << "Expected 32, got " << len << ".";
  + triggerJavaIllegalStateException(env, err.str().c_str());
  + return nullptr;
  }
  assert(len == 32);
  