Add version token to error URL

Add version token to error URL

the sqlalche.me redirector now supports the numerical version
code in the URL, e.g. /13/, /14/, /20/, etc., so that we can
redirect to the error codes for the appropriate version
of SQLAlchemy in use without going through the catch-all "latest"
link.   If a particular version of the docs is no longer on the
site, the redirect will revert to falling through the "latest"
link (which ultimately lands on the current release version,
/13/ at the time of this writing).

Change-Id: I3bb463fd6fb6c8767c95a57f3699aba715a9a72d