RFR: 1707: Bot warns about "No .jcheck/conf found" when it should not [v3]

[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"

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  make HostedRepository::fileContents return Optional

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1435/files
  - new: https://git.openjdk.org/skara/pull/1435/files/f5c68229..df16b511

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1435&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1435&range=01-02

  Stats: 83 lines in 16 files changed: 26 ins; 13 del; 44 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