RFR: sync: catch exceptions when creating Forge

[Erik Helin]

Hi all,

please review this patch that makes `git-sync` prints a more informative error message when the `Forge` can't be
instantiated when trying to auto-magically figure out the "upstream" repository.

Thanks,
Erik

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Commit messages:
 - sync: catch exceptions when creating Forge

Changes: https://git.openjdk.java.net/skara/pull/779/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=779&range=00
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/779.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/779/head:pull/779

PR: https://git.openjdk.java.net/skara/pull/779