Wrong error message in HDFStore.append
Updated pytables.py to clarify error message when appending dataframe with None item in previously string-only column

- [ ] closes #16300
- [ ] tests passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@bryanfree66 can you run the issue case, and report the new error message please?

```
store = pd.HDFStore('teststore.h5', 'w')
chunk = pd.DataFrame({'V1':['a','b','c','d','e'], 'data': range(5)})
store.append('df', chunk)
chunk = pd.DataFrame({'V1':[None, None], 'data': [3, 5]})
store.append('df', chunk)
``` >>> import pandas as pd
>>> store = pd.HDFStore('teststore.h5', 'w')
>>> chunk = pd.DataFrame({'V1':['a','b','c','d','e'], 'data': range(5)})
>>> store.append('df', chunk)
>>> chunk = pd.DataFrame({'V1':[None, None], 'data': [3, 5]})
>>> store.append('df', chunk)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 988, in append
    **kwargs)
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 1415, in _write_to_group
    s.write(obj=value, append=append, complib=complib, **kwargs)
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 3996, in write
    **kwargs)
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 3667, in create_axes
    raise e
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 3662, in create_axes
    info=self.info)
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 2001, in set_atom
    errors)
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 2048, in set_atom_string
    eci = existing_col.validate_col(itemsize)
  File "/Users/bryan/code/pandas/pandas/io/pytables.py", line 1739, in validate_col
    c.itemsize))
ValueError: Trying to store a string with len [3] in the column [V1], but
this column has a limit of [1]!
Consider using min_itemsize to preset the sizes on these columns needs a test ```
ValueError: Trying to store a string with len [3] in the column [V1], but
this column has a limit of [1]!
Consider using min_itemsize to preset the sizes on these columns
```
I don't think this error is correct. The example doesn't try to store a trying with len 3, is trying to store a `None`. Isn't that right? Yeah, I noticed another issue too. I’m going to cancel this PR and work on fixing it properly.
On Aug 24, 2018, 9:22 AM -0500, Marc Garcia <notifications@github.com>, wrote:
> ValueError: Trying to store a string with len [3] in the column [V1], but
> this column has a limit of [1]!
> Consider using min_itemsize to preset the sizes on these columns
> I don't think this error is correct. The example doesn't try to store a trying with len 3, is trying to store a None. Isn't that right?
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub, or mute the thread.
