IOW if we ended up in vcl_backend_error{} or failed after returning
delivery in vcl_backend_response{} (e.g. we did not receive enough bytes)
we will bump this counter.

Conflicts:
	bin/varnishd/cache/cache_fetch.c
	bin/varnishtest/tests/r01624.vtc
	include/tbl/vsc_f_main.h