When trying to delete a non-existing route (such as "foo" below), one
gets an
incomprehensible error message:

$ sudo route del foo
SIOCDELRT: No such process

I have no idea what SIOCDELRT means, and even if I look that up, the "No
such process" makes no sense to me, because I wanted to delete a route,
not a process.

Something like 'No such route: "foo"' would be much better and more
informative.