Minor improvements to NetworkController error reporting.

Currently, when trying to perform an operation on netId that
does not exist, we return EINVAL. This can be confusing
because lots of things can return EINVAL for many different
reasons.

Instead, change this to ENONET ("Machine is not on network"),
which was what Sreeram originally implemented before we changed
it to EINVAL. I also considered ENOENT ("No such file or
directory"), but on reflection that seems less appropriate, and
it's used by other things as well. ENONET appears to have no
uses in our tree. It is also clearly separate from the ESRCH we
return for nonexistent rules.

While I'm at it, also disambiguate some of the
if (foo || bar ) { return EFOO; } to return two different error
codes.

Bug: 16667349
Change-Id: Ief2d720a58679aa746f5ba273d545487d0825e52