Fix varnish check for Bearer token presence

Varnish has an undocumented acceptable range of values that can be
passed as the code to the error built-in function. 1401 is outside of
that range.

See https://github.com/varnish/Varnish-Cache/blob/5cf517e858b212a37a106a8ae78ffabe13a0dd5b/bin/varnishd/cache/cache_vrt.c#L62-L63
Merge pull request #292 from alphagov/fix-varnish-bearer-token-check

Fix varnish check for Bearer token presence