Crash while checkpointing due to fs.* errors

If a `.city` file is removed while the server is preparing to checkpoint, it will cause the server to crash before the checkpoint is performed:

```
Checkpoint 2021-03-02T11.48.19.274Z.city complete.
Checkpointing...
internal/fs/utils.js:220
throw err;
^

Error: ENOENT: no such file or directory, stat '2021-03-02T10.11.40.306Z.city'
at Object.statSync (fs.js:915:3)
at Object.CodeCity.fileSize (/Users/cpcallen/src/CodeCity/server/codecity:286:13)
at Object.CodeCity.deleteCheckpointsIfNeeded (/Users/cpcallen/src/CodeCity/server/codecity:207:16)
at Timeout.CodeCity.checkpoint [as _onTimeout] (/Users/cpcallen/src/CodeCity/server/codecity:296:12)
at listOnTimeout (internal/timers.js:531:17)
at processTimers (internal/timers.js:475:7) {
errno: -2,
syscall: 'stat',
code: 'ENOENT',
path: '2021-03-02T10.11.40.306Z.city'
}
```

The checkpointing code should have some `try {…} catch {…}`es added so that it will not have a fit if a file disappears out from it or there are otherwise errors (e.g. in writing).

Just not checkpointing at all is preferable to crashing.