R1:356b2781bcad

Gracefully fail request if non existing callsign is passed to getrecentcommitsbypath instead of crashing

Summary:
`diffusion.getrecentcommitsbypath` fails with 500 error when non existing callsign is passed:
```
>>> UNRECOVERABLE FATAL ERROR <<<

Call to a member function getCommit() on null

```

Expected Behavior:
Return more graceful error notifying caller that such callsign/repository does not exist

Reproduction steps:
Open conduit: https://secure.phabricator.com/conduit/method/diffusion.