HTML-escape error messages (CVE-2016-4561)
The instance in cgierror() is a potential cross-site scripting attack,
because an attacker could conceivably cause some module to raise an
exception that includes attacker-supplied HTML in its message, for
example via a crafted filename. (OVE-
20160505-0012, CVE-2016-4561)
The instances in preprocess() is just correctness. It is not a
cross-site scripting attack, because an attacker could equally well
write the desired HTML themselves; the sanitize hook is what
protects us from cross-site scripting here.