Raise CuPy not available error early in cuda.GpuDevice initialization

chainer/backends/cuda.py

@@ -158,8 +158,8 @@ def __ne__(self, other):
 class GpuDevice(_backend.Device):
 
     def __init__(self, device):
-        assert isinstance(device, Device)
         check_cuda_available()
+        assert isinstance(device, Device)
 
         super(GpuDevice, self).__init__()
         self.device = device