replace isinstance with separate except block

[mercutiodesign]

Instead of testing if an exception is a HTTPException with isinstance, I would propose to just add a dedicated except block for this

[slingamn]

Thanks for the PR! I had it like this before and it produced a Pylint W0706. However, it's not important anymore that the project pass Pylint (I envision anyone vendoring it will have Pylint ignore the entire file), so I'll reconsider this.