Don't crash on filters testing a non-existent link-layer type field.

Instead, generate an error indicating that the link-layer type filtering
isn't implemented for that link-layer header type, just as we already do
for some types.  That way, we don't punish programs, or their users, for
not knowing (or caring) that a given link-layer header type doesn't have
a way to determine the protocol running atop that link-layer header
type.
Note the most recent change.
In case there's ever a 1.6.2 (or somebody builds from Git).