tests/netgraph: Attribute errors to the caller

[donner]

Summary

Errors raised in the common util functions should raise the location
of their caller to be useful and include the errno description.

Depends on D30629

Test Plan

Before

failed: .../util.c:151: -1 != NgSendData(ds, hook, data, len) not met

After

failed: .../basic.c:55: -1 != NgSendData(ds, hook, data, len) not met (No route to host)