[Varnish] #1840: add beresp.backend.name field to vcl_error

[Varnish]

#1840: add beresp.backend.name field to vcl_error
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 Reporter:  jzinner      |       Owner:
     Type:  enhancement  |      Status:  new
 Priority:  normal       |   Milestone:
Component:  build        |     Version:  unknown
 Severity:  normal       |  Resolution:
 Keywords:               |
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Comment (by jzinner):

 Also req.backend (available during vcl_recv) is not enough since I have a
 very large number of backends under a director ~ 120 hosts

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1840#comment:1>
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