[BazelBot] Gracefully fail when we error out on merging

[boomanaiden154]

Traceback (most recent call last):
  File "/var/lib/buildkite-agent/bazel-fixer-bot/./bazelbot_server_main.py", line 63, in <module>
    bot.run()
  File "/var/lib/buildkite-agent/bazel-fixer-bot/bazelbot_server.py", line 230, in run
    ] = self.build_processor.get_builds_to_process(self.last_processed_sha)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/buildkite-agent/bazel-fixer-bot/utils.py", line 339, in get_builds_to_process
    self.git_repo.refresh_main_branch()
  File "/var/lib/buildkite-agent/bazel-fixer-bot/utils.py", line 216, in refresh_main_branch
    self.gh_fork_repo.merge_upstream(self.main_branch)
  File "/var/lib/buildkite-agent/bazel-fixer-bot/.venv/lib/python3.12/site-packages/github/Repository.py", line 3953, in merge_upstream
    headers, data = self._requester.requestJsonAndCheck("POST", f"{self.url}/merge-upstream", input=post_parameters)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/buildkite-agent/bazel-fixer-bot/.venv/lib/python3.12/site-packages/github/Requester.py", line 623, in requestJsonAndCheck
    return self.__check(
           ^^^^^^^^^^^^^
  File "/var/lib/buildkite-agent/bazel-fixer-bot/.venv/lib/python3.12/site-packages/github/Requester.py", line 853, in __check
    raise self.createException(status, responseHeaders, data)
github.GithubException.GithubException: Server Error: 502 {"message": "Server Error"}

Still can't quite figure out why we would be getting a 502 here. The server restarted just fine and was able to run this command just fine after restarting.