Account for the fact that comments might get deleted while-or-just-before we process them

[cvrebert]

03:00:33.557 [on-spray-can-akka.actor.default-dispatcher-25523] ERROR com.jcabi.github.Comment$Smart - #url(): thrown java.lang.AssertionError(HTTP response status is not equal to 200:
404 Not Found [https://api.github.com/repos/twbs/bootstrap/issues/comments/136919469]

[cvrebert]

at com.jcabi.github.Comment$Smart.url
    (Comment.java:158) ~[server.jar:1.0]
at com.chrisrebert.lmvtfy.github.GitHubIssueCommenter$$anonfun$receive$1.applyOrElse
    (GitHubIssueCommenter.scala:44) ~[server.jar:1.0]