chore(all): review error wrappings

- remove repetitive `cannot` and `failed` prefixes
- rename `unmarshaling` to `decoding`

internal/wireguard/netlink_integration_test.go

@@ -117,5 +117,5 @@ func Test_netlink_Wireguard_addRule(t *testing.T) {
 		_ = nilCleanup() // in case it succeeds
 	}
 	require.Error(t, err)
-	assert.Equal(t, "cannot add rule ip rule 10000: from all to all table 999: file exists", err.Error())
+	assert.EqualError(t, err, "adding rule ip rule 10000: from all to all table 999: file exists")
 }