RFR: 2295: Rephrase error messages in PullRequestCheckIssueVisitor

[Zhao Song]

PullRequestCheckIssueVisitor is responsible for generating error messages when jcheck fails. Currently, jcheck could be configured as different severity(error or warning). However, when generating error messages for some jchecks like binary check, the error message would be like "Binary files are not allowed", so if the binary check was configured as warning, this message seems too hard. Therefore, I would like to rephrase some error messages.

Erik suggested that we should generate different messages depending on the severity, but I think maybe it's unnecessary. The user does not need to judge the severity depending on the error message. The user only needs to check which section is the message under(Integration Blocker or Warning).

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Commit messages:
 - fix test
 - update
 - SKARA-2295

Changes: https://git.openjdk.org/skara/pull/1660/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1660&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2295
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1660.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1660/head:pull/1660

PR: https://git.openjdk.org/skara/pull/1660