RFR: forge: GitLab members endpoint return 404 for non-members

[Erik Helin]

Hi all,

please review this patch that fixes how `GitLabRepository.canPush` uses the `members` REST endpoint. Turns out that the `members` endpoint returns a `404` if the user is _not_ a member of the project (not documented of course �� ).

Thanks,
Erik

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Commit messages:
 - forge: GitLab members endpoint return 404 for non-members

Changes: https://git.openjdk.java.net/skara/pull/1106/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1106&range=00
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1106.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1106/head:pull/1106

PR: https://git.openjdk.java.net/skara/pull/1106