Fix some more flake8 violations

[graingert]

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10038
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I ran additional checks listed at Getting Your Patch Accepted
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• I have updated the automated tests.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.

[adiroiban]

Looks good.

The use BaseException everywhere in this PR is a safe change.

But I feel that we are cheating the flake8 warning...as my understanding is that the flake8 is designed to make sure use a higher level exception and don't also catch SystemExit and other base exceptions.

But I think that this is moving into the right direction and in the future we will see more explicit exception handling

Thanks. Great work! Happy to see less flake8 things ignored.