gh-ost doesn't mind if you purge binary logs....

I'm not sure if this is a bug or gh-ost just being polite. However, I was running a gh-ost migration where disk space was at a premium, so I was periodically going through and purging binary logs along the replication chain so I wouldn't run out of disk space. Right after I pressed on purging the master binary logs, I had the realization that I had just broken the migration. I checked the output of gh-ost and saw that it was on binary log `mysql-bin.004134` and I had just purged to `mysql-bin.004182`. Now, gh-ost, being the polite program it was, it realized it had reached the "end" of 4134, and continued to "read" 4135, which it couldn't find, so it moved on to the next binary and the next until it got to 4182 and continued running from there .No error message, just messages like:
```
2018-08-17 18:49:58 INFO rotate to next log name: mysql-bin.004181
2018-08-17 18:49:58 INFO rotate to next log name: mysql-bin.004181
```

If gh-ost shouldn't bail out in these cases, it should at least scream loudly that it could not read the file.

cc: https://github.com/github/hookshot/pull/930