sd: "no paths found" logging is opaque

When sciond can't find any paths:
```
2019-10-31 14:26:07.868232+0000 [EROR] No convergence in lookup debug_id=42f0a1eb iteration=3
2019-10-31 14:26:07.868241+0000 [EROR] Unable to get paths debug_id=42f0a1eb err="Segment lookup doesn't converge iterations="3""
```
"convergence" isn't something a user will understand (i certainly don't know what it means from looking at the logs).