Show more readable error message when uploading gist or posting comment failed.

[Flamefire]

We can catch HTTPErrors and convert it to a EasyBuildError instead of letting it show up in main.
For the common 401 error indicating a token issue we can add a more useful message.

Without it it looks like this:

EasyBuild crashed! Please consider reporting a bug, this should not happen...

Traceback (most recent call last):
  File "/usr/lib64/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib64/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/easybuild-framework/easybuild/main.py", line 843, in <module>
    main_with_hooks()
  File "/easybuild-framework/easybuild/main.py", line 829, in main_with_hooks
    sys.exit(int(main(args=args, prepared_cfg_data=(init_session_state, eb_go, cfg_settings))))
  File "/easybuild-framework/easybuild/main.py", line 780, in main
    is_successful = process_eb_args(orig_paths, eb_go, cfg_settings, modtool, testing, init_session_state,
  File "/easybuild-framework/easybuild/main.py", line 614, in process_eb_args
    test_report_msg = overall_test_report(ecs_with_res, len(paths), overall_success, success_msg, init_session_state)
  File "/easybuild-framework/easybuild/tools/testing.py", line 459, in overall_test_report
    txt = post_pr_test_report(pr_nrs, GITHUB_EASYCONFIGS_REPO, test_report, msg, init_session_state,
  File "/easybuild-framework/easybuild/tools/testing.py", line 362, in post_pr_test_report
    gist_url = upload_test_report_as_gist(test_report['full'], descr=descr, fn=fn)
  File "/easybuild-framework/easybuild/tools/testing.py", line 335, in upload_test_report_as_gist
    gist_url = create_gist(test_report, descr=descr, fn=fn, github_user=github_user)
  File "/easybuild-framework/easybuild/tools/github.py", line 839, in create_gist
    status, data = g.gists.post(body=body)
  File "/easybuild-framework/easybuild/base/rest.py", line 140, in post
    return self.request(self.POST, url, json.dumps(body), headers, content_type='application/json')
  File "/easybuild-framework/easybuild/base/rest.py", line 177, in request
    conn = self.get_connection(method, url, body, headers)
  File "/easybuild-framework/easybuild/base/rest.py", line 216, in get_connection
    connection = self.opener.open(request)
  File "/usr/lib64/python3.9/urllib/request.py", line 523, in open
    response = meth(req, response)
  File "/usr/lib64/python3.9/urllib/request.py", line 632, in http_response
    response = self.parent.error(
  File "/usr/lib64/python3.9/urllib/request.py", line 561, in error
    return self._call_chain(*args)
  File "/usr/lib64/python3.9/urllib/request.py", line 494, in _call_chain
    result = func(*args)
  File "/usr/lib64/python3.9/urllib/request.py", line 641, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 401: Unauthorized