#23648 - Validate docstring now checks for existence of docstring first.
TST - validate_one() will check if docstring exists before performing other checks. If it does not exist, then a single error will be returned reading 'Docstring does not exist'
PERF - validate_one will skip other validation checks if docstring doesn't exist, which will save time in cases of high volumes of function checks without docstrings.

- [ ] closes #23648 
- [ ] passes `python3 ./scripts/validate_docstrings.py --prefix=pandas.Series.imag`
- [ ] whatsnew Smart validation checking in validate_docstrings
<img width="767" alt="screen shot 2018-11-13 at 10 58 12 am" src="https://user-images.githubusercontent.com/12774588/48425555-11a25600-e733-11e8-8b0e-c481244b4b01.png">
