RFR: 1707: Bot warns about "No .jcheck/conf found" when it should not

[Zhao Song]

In [SKARA-1393](https://bugs.openjdk.org/browse/SKARA-1393), I added some checks to ensure the target branch of a pr contains valid jcheck configuration. This change went live on November 28, however, some users found that the warning of "No .jcheck/conf found" printed after they integrated their pr. After more investigation, I am thinking it's maybe a GitLab bug. Since when the issue happens, GitLab will always return "Commit Not Found", so we could have a temporary workaround right now.

For GitLab REST API:
If commit not found, it will return "404 Commit Not Found"

If file not found it will return "404 File Not Found"

For Github REST API:
If commit not found, it will return "No commit found for the ref "

If file not found it will return "Not found"

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Commit messages:
 - SKARA-1707

Changes: https://git.openjdk.org/skara/pull/1435/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1435&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1707
  Stats: 24 lines in 5 files changed: 15 ins; 2 del; 7 mod
  Patch: https://git.openjdk.org/skara/pull/1435.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1435/head:pull/1435

PR: https://git.openjdk.org/skara/pull/1435