routing: modify TestSendToRouteStructuredError to return non-second chance error
routing: modify TestSendToRouteStructuredError to return non-second chance error

In this commit, we modify the existing `TestSendToRouteStructuredError`
test to return an error that doesn't trigger the second chance logic.
Otherwise, we'll get a nil failure result from the mission control
interpretation, meaning we won't exercise the full code path. Instead,
we use a terminal error to ensure that the expected code path is hit.

As is, this test will fail as a recent refactoring causes us to return a
`channeldb.FailureReason` error, since the newly added `handleSendError`
code path in the `SendToRoute` method will return the raw error, rather
than the `shardError`, which is of the expected type.

aakselrod/lnd,aakselrod/lnd