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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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$ make # see install targets
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$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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