ER: remove all calls to str in error messages
```
In [9]: df = mkdf(10, 10, data_gen_f=lambda *args: randint(2), c_idx_type='u', r_idx_type='u')

In [10]: df.sort_index(df.columns[0])
```

barfs a `UnicodeEncodeError` inside of a `ValueError`. this maybe ongoing, but might be easy enough to use a macro to correct this in a reasonably fast way
