[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  |       Type:  enhancement
   Status:  new      |   Priority:  normal
Milestone:           |  Component:  build
  Version:  unknown  |   Severity:  normal
 Keywords:           |
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 It would be nice to have access to something like beresp.backend.name from
 vcl_error -- so that incase a backend fails; I can return more detail in
 the error, such as the host that failed.

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