Get better diagnostics on ESI-code string errors.
Get better diagnostics on ESI-code string errors.

Related to: #1773

franciscovg/Varnish-Cache,gquintard/Varnish-Cache,franciscovg/Varnish-Cache,varnish/Varnish-Cache,gquintard/Varnish-Cache,franciscovg/Varnish-Cache,varnish/Varnish-Cache,chrismoulton/Varnish-Cache,chrismoulton/Varnish-Cache,chrismoulton/Varnish-Cache,varnish/Varnish-Cache,gquintard/Varnish-Cache,ajasty-cavium/Varnish-Cache,ajasty-cavium/Varnish-Cache,feld/Varnish-Cache,ajasty-cavium/Varnish-Cache,chrismoulton/Varnish-Cache,feld/Varnish-Cache,zhoualbeart/Varnish-Cache,zhoualbeart/Varnish-Cache,zhoualbeart/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,feld/Varnish-Cache,zhoualbeart/Varnish-Cache,gquintard/Varnish-Cache,chrismoulton/Varnish-Cache,zhoualbeart/Varnish-Cache,varnish/Varnish-Cache,ajasty-cavium/Varnish-Cache,franciscovg/Varnish-Cache,ajasty-cavium/Varnish-Cache,franciscovg/Varnish-Cache,feld/Varnish-Cache