tests/test-check-jshint.t

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

#require test-repo jshint hg10

  $ . "$TESTDIR/helpers-testrepo.sh"

run jshint on all tracked files ending in .js except vendored dependencies

  $ cd "`dirname "$TESTDIR"`"

  $ testrepohg locate 'set:**.js' \
  > 2>/dev/null \
  > | xargs jshint