ERR: validation options that accept callables
https://github.com/pydata/pandas/issues/12704#issuecomment-200554983

```
In [1]: pd.describe_option('float_format')
display.float_format : callable
    The callable should accept a floating point number and return
    a string with the desired format of the number. This is used
    in some places like SeriesFormatter.
    See core.format.EngFormatter for an example.
    [default: None] [currently: None]
```

This should raise

```
In [2]: pd.set_option('float_format','%.1f')
```
