Show negative results to Pull-Requests details

Negative results logged with a "!!" prefix.

Updates #95

$ go run . --repo=github.com/ossf/scorecard --show-details --checks=Pull-Requests
Starting [Pull-Requests]
Finished [Pull-Requests]

RESULTS
-------
Pull-Requests: Pass 9
    !! found commit without PR: 71dace5, committer: dlorenc
    found PRs for 29 out of 30 commits

checks/pull_requests.go

@@ -62,6 +62,8 @@ func PullRequests(c checker.Checker) checker.CheckResult {
 		}
 		if len(prs) > 0 {
 			totalWithPrs++
+		} else {
+			c.Logf("!! found commit without PR: %s, committer: %s", commit.GetSHA(), commit.GetCommitter().GetLogin())
 		}
 	}
 	c.Logf("found PRs for %d out of %d commits", totalWithPrs, total)