arrow

ARROW-1454: [Python] Also match ArrowNotImplementedError in unsupported type conversions from pandas

Unsupported conversions were bubbling up to pandas users in an unfriendly way. Now we have:

```
In [1]: paste
import pandas as pd
import numpy as np
import pyarrow as pa

df = pd.DataFrame({
    't1': pd.date_range('2000-01-01', periods=20),
    't2': pd.date_range('2000-05-01', periods=20)
})
df['diff'] = df.t2 - df.t1
pa.Table.from_pandas(df)

## -- End pasted text --
<SNIP>
~/code/arrow/python/pyarrow/error.pxi in pyarrow.lib.check_status()
     87             raise ArrowKeyError(message)
     88         elif status.IsNotImplemented():
---> 89             raise ArrowNotImplementedError(message)
     90         elif status.IsTypeError():
     91             raise ArrowTypeError(message)

ArrowNotImplementedError: ('Unsupported numpy type 22\n', 'Conversion failed for column diff with type timedelta64[ns]')
```

Author: Wes McKinney <wesm+git@apache.org>

Closes #2227 from wesm/ARROW-1454 and squashes the following commits:

673eb9cf <Wes McKinney> Also match ArrowNotImplementedError in unsupported type conversions from pandas