segfetcher: add debug log for destination unreachable errors
segfetcher: add debug log for destination unreachable errors

GitOrigin-RevId: 78aba430cf0c6052546061954f11e34d5398f818

netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion