Improve error reporting

[jcfr]

This would make it easier to understand the issue with failing build like this one: https://circleci.com/gh/hdmf-dev/hdmf/423
where the error was like:

Traceback (most recent call last):
  File "/home/circleci/venv/bin/githubrelease", line 11, in <module>
    sys.exit(main())
  File "/home/circleci/venv/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/circleci/venv/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/circleci/venv/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/circleci/venv/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/circleci/venv/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/circleci/venv/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/circleci/venv/lib/python3.7/site-packages/click/decorators.py", line 27, in new_func
    return f(get_current_context().obj, *args, **kwargs)
  File "/home/circleci/venv/lib/python3.7/site-packages/github_release.py", line 437, in cli_release_create
    gh_release_create(*args, **kwargs)
  File "/home/circleci/venv/lib/python3.7/site-packages/github_release.py", line 116, in with_check_for_credentials
    return func(*args, **kwargs)
  File "/home/circleci/venv/lib/python3.7/site-packages/github_release.py", line 463, in gh_release_create
    response.raise_for_status()
  File "/home/circleci/venv/lib/python3.7/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://api.github.com/repos/hdmf-dev/hdmf/releases

In that particular case, the bot didn't have access to the repo.

Thanks @ajtritt for reporting the problem.