RFR: forge: GitHubRepository.canPush might return 404

[Erik Helin]

Hi all,

please review this patch that makes `GitLabRepository.canPush` handle when the request returns a `404`. GitHub returns `404` when a user is not a collaborator in a repository.

Thanks,
Erik

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

Commit messages:
 - forge: GitHubRepository.canPush might return 404

Changes: https://git.openjdk.java.net/skara/pull/1137/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1137&range=00
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1137.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1137/head:pull/1137

PR: https://git.openjdk.java.net/skara/pull/1137