no error when getting a variable that doesn't exist in current step

Created by: germasch

To reproduce:

TEST_F(ADIOS2_CXX11_API_IO, StreamingWrite)
{
    adios2::Engine writer = m_Io.Open("xxx.bp", adios2::Mode::Write);

    double x[3];
    auto var = m_Io.DefineVariable<double>("x", {3}, {0}, {3});
    for (int step = 0; step < 3; step++) {
      x[0] = 10 + step;
      x[1] = 20 + step;
      x[2] = 30 + step;
      writer.BeginStep();
      if (step > 0) {
	writer.Put(var, x);
      }
      writer.EndStep();
    }
    writer.Close();
}

TEST_F(ADIOS2_CXX11_API_IO, StreamingRead)
{
    adios2::Engine reader = m_Io.Open("xxx.bp", adios2::Mode::Read);

    double x[3];
    auto var = m_Io.InquireVariable<double>("x");
    for (int step = 0; step < 3; step++) {
      reader.BeginStep();
      reader.Get(var, x);
      reader.EndStep();
      printf("step %d: %g %g %g\n", step, x[0], x[1], x[2]);
    }
    reader.Close();
}

The second test, when run, gives

[ RUN      ] ADIOS2_CXX11_API_IO.StreamingRead
step 0: 6.95312e-310 2.19424e-314 5.2286e-310
step 1: 11 21 31
step 2: 12 22 32
[       OK ] ADIOS2_CXX11_API_IO.StreamingRead (2 ms)

The file doesn't contain the variable x in step 0, so I don't expect it to read anything (ie, non-initialized nonsense in x is fine), but I do expect to get some kind of an error when the Get call does not succeed.