Give a better error when using the old GPU back-end.

theano/sandbox/cuda/__init__.py

+raise RuntimeError(
+    "You are importing theano.sandbox.cuda. This is the old GPU back-end and "
+    "is removed from Theano. Use Theano 0.9 to use it. Even better, "
+    "transition to the new GPU back-end! See "
+    "https://github.com/Theano/Theano/wiki/Converting-to-the-new-gpu-back-end%28gpuarray%29")