We wrap VAS_Fail() as VPI_Fail() and add an assert() macro to the
Varnish Generated C (VGC) code. This assert() definition is basically
a copy of that in vas.h, but deliberately added to a source file which
is only used by generate.py for VGC, such that this slightly different
definition be only visible to VGC.

I also pondered the option to include VCL source information in
VPI_Fail: By using the information updated by VPI_count(), we could
have that, but as the assert is, for now, intended only to ensure
correctness of VCC and core VRT code, I decided against this
complication.

With assert() in place for VGC, we arm the assertions from
75acb5cc