RFR: 2307: PullRequestCheckIssueVisitor generates only one error message per Issue type

[Zhao Song]

When testing [SKARA-2303](https://bugs.openjdk.org/browse/SKARA-2303), I found that even when there are few binary files in a diff, the pr body only warns about one binary file. Then I found that in PullRequestCheckIssueVisitor, we are using HashMap<Check, String> to store the error messages, so if there are few issues with the same type, the HashMap is only able to save the last error message.

-------------

Commit messages:
 - SKARA-2307

Changes: https://git.openjdk.org/skara/pull/1662/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1662&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2307
  Stats: 26 lines in 2 files changed: 8 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/skara/pull/1662.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1662/head:pull/1662

PR: https://git.openjdk.org/skara/pull/1662