RFR: 1707: Warning of "No .jcheck/conf found" printed when there is no such problem actually

[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 orahub bug and filed an issue to orahub(https://jira-sd.mc1.oracleiaas.com/browse/ORAHUB-1638). Since when the issue happens, orahub will always return "Commit Not Found", so we could have a temporary workaround right now.

For Orahub 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 <refname>"

If file not found it will return "Not found"

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Commit messages:
 - SKARA-1707

Changes: https://git.openjdk.org/skara/pull/1434/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1434&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/1434.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1434/head:pull/1434

PR: https://git.openjdk.org/skara/pull/1434