BUG: fix impl check in RayTransform

Author: kohr-h

odl/tomo/operators/ray_trafo.py

@@ -144,6 +144,9 @@ def __init__(self, reco_space, geometry, variant, **kwargs):
                         RuntimeWarning)
             else:
                 raise RuntimeError('bad impl')
+
+            impl_in = None
+
         else:
             impl, impl_in = str(impl).lower(), impl
             if impl not in _SUPPORTED_IMPL:
@@ -157,8 +160,15 @@ def __init__(self, reco_space, geometry, variant, **kwargs):
         # Sanity checks
         if impl.startswith('astra'):
             if geometry.ndim > 2 and impl.endswith('cpu'):
-                raise ValueError('`impl` {!r} only works for 2d'
-                                 ''.format(impl_in))
+                if impl_in is None:
+                    raise ValueError(
+                        'fastest available `impl` {!r} only supports 2D'
+                        ''.format(impl)
+                    )
+                else:
+                    raise ValueError(
+                        '`impl` {!r} only supports 2D'.format(impl_in)
+                    )
 
             # Print a warning if the detector midpoint normal vector at any
             # angle is perpendicular to the geometry axis in parallel 3d