errors: introduce SecurityError and use it in a few places
This is part of
https://www.mercurial-scm.org/wiki/ErrorCategoriesPlan. There are
perhaps more errors in `sslutil.py` that should raise `SecurityError`;
I picked the most clear ones to start with.
Differential Revision: https://phab.mercurial-scm.org/D9390
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true