Previously, we had zero stats on the cause of backend connection
errors, which made it close to impossible to diagnose such issues in
retrospect (only via log mining).

We now pass an optional backend vsc to vcp and record errors per
backend.

Open errors are really per vcp entry (ip + port or udc path), which
can be shared amongst backends (and even vcls), but we maintain the
counters per backend (and, consequently, per vcl) for simplicity.  It
should be noted though that errors for shared endpoints affect all
backends using them.

Ref #2622