Don't throw exceptions

- Return np.nan instead
- As per behavior of whaam

src/cr/cube/crunch_cube.py

@@ -821,9 +821,8 @@ def _mr_index(self, axis, weighted):
 
         if self.dimensions[0].type == 'multiple_response':
             if axis != 0:
-                raise ValueError(
-                    'MR x CAT index table only defined for column direction'
-                )
+                # MR x CAT index table only defined for column direction.
+                return np.full(self.as_array().shape, np.nan)
             selected = (
                 table[:, 0, :, 0]
                 if self.is_double_mr else
@@ -840,9 +839,9 @@ def _mr_index(self, axis, weighted):
 
         if self.dimensions[1].type == 'multiple_response':
             if axis == 0:
-                raise ValueError(
-                    'CAT x MR index table not defined for column direction'
-                )
+                # CAT x MR index table not defined for column direction.
+                return np.full(self.as_array().shape, np.nan)
+
             selected = table[:, :, 0]
             non_selected = table[:, :, 1]
             margin = np.sum(selected, 0) / np.sum(selected + non_selected, 0)