Yes, the specific error message does matter, because it will indicate what, specifically, is going wrong.
But, no, D3D9 is not sufficient to run UE4. Most computers these days support DX11 anyway – even the built-in Intel graphics stuff has supported that for a long time.
In fact, a built-in Intel graphics chip is likely much faster than the 7900GS. That card is over 10 years old! It only has 256 MB of graphics RAM and 20 pixel shaders (supporting shader model 3) which is roughly equivalent to Intel HD 4400 graphics. Except HD 4400 supports D3D11.