RFR: 2059: Improve error message when cli encounters 401

[Zhao Song]

As Erik said in the issue "A user tried to run `git pr checkout` and got UncheckedRestException with 401. The repository in question had remotes pointing to a gitlab instance while the user was trying to get a PR from github. Not sure if that is relevant here. We should not fail with a stacktrace but should rather explain that we authorization failed and print which URL we tried to connect to."

To not fail with a stack trace, in this patch, `UncheckedRestException` will be caught in the `GitSkara#main` and status code will be checked, if the status code is 401, the cli tool should print suitable error message to the user.

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Commit messages:
 - update
 - SKARA-2059

Changes: https://git.openjdk.org/skara/pull/1571/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1571&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2059
  Stats: 35 lines in 5 files changed: 22 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/skara/pull/1571.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1571/head:pull/1571

PR: https://git.openjdk.org/skara/pull/1571