BUG: Throw a ParserError when header rows have unequal column counts …
(GH43102)

- closes #43102 
- Fixed uncaught IndexError that is raised when the iterator `i` in `extract(r)` inside base_parser.py exceeds the length of a header row (when `field_count > len(r)`).
- A Parser error is raised when the header row had unequal columns. Outputs the first row where a mismatch is found.
- Added a test to match the error being raised.
Hello @quantumalaviya! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-09-05 08:27:17 UTC @jreback I added a test inside pandas/tests/io/parser/test_header.py. Is this fine? I had added "A Parser error is raised when the header row had unequal columns. Outputs the first row where a mismatch is found." as a substitute for whatsnew. Do you want it to be more detailed or am I missing something? Please add something about the uncaught error before to avoid the impression that a new error is raised @jreback @phofl Does this look good? Please be patient, we are all volunteers and will look when we have time.

Whatsnew is still missing Thanks for the clarification, I was confused as to what the next step is. 

EDIT: Changed whatsnew. Yes the whatsnews are our release notes. Small comment, otherwise lgtm @lithomas1 merge away thanks @quantumalaviya  My pleasure. Thanks for guiding me through it!