Column: Only fail when an invalid UDA type is found

- Previously it was failing for a missing type, so that if a UDA was not
  completely defined, there was an unexpected error. This is because the
  setting uda.foo.label was enough to make Column extract 'foo', but then
  fail to extract uda.foo.type.

src/columns/Column.cpp

@@ -216,8 +216,9 @@ Column* Column::uda (const std::string& name)
       split (c->_values, values, ',');
     return c;
   }
+  else if (type != "")
+    throw std::string (STRING_UDA_TYPE);
 
-  throw std::string (STRING_UDA_TYPE);
   return NULL;
 }