title: using pytest.raises to catch expected custom error
body: <p>I am new to <code>pytest</code> and am trying to convert some functional test scripts into ones that play nicely with <code>pytest</code>. My module has custom error types and I'm trying to use the <code>with pytest.raises() as excinfo</code> method. This is a scientific/numerical package and I need to test that certain methods are consistent when called, so I can't just drill down to the lower-level stuff.</p>