git sync fails badly, not goodly, when branch is unpublished

I am using legit version 1.2.0post0 installed from pip. When I have a branch that is unpublished, git sync fails badly:

lousyd $ git status
On branch 174-branchname
Your branch is up to date with 'origin/174-branchname'.

nothing to commit, working tree clean

lousyd $ git branches
   172-some-other-branch                  (published)    
*  174-branchname                         (unpublished)  
   179-another-branch                     (published)    

lousyd $ git pull --all
Fetching origin
Your configuration specifies to merge with the ref 'refs/heads/174-branchname'
from the remote, but no such ref was fetched.

lousyd $ git sync
Pulling commits from the server.
Traceback (most recent call last):
  File "/home/lousyd/.local/bin/legit", line 8, in <module>
    sys.exit(cli())
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/decorators.py", line 73, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/legit/cli.py", line 167, in sync
    status_log(scm.smart_pull, 'Pulling commits from the server.')
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/legit/utils.py", line 14, in status_log
    log = func(*args, **kwargs)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/legit/scm.py", line 137, in smart_pull
    self.smart_merge_enabled())
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/legit/scm.py", line 151, in smart_merge
    ['log', '--merges', '{}..{}'.format(branch, from_branch)])
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/legit/scm.py", line 60, in git_exec
    result = self.repo.git.execute(command, **kwargs)
  File "/home/lousyd/.local/share/virtualenvs/pip-fh2szxJc/lib/python3.6/site-packages/git/cmd.py", line 823, in execute
    raise GitCommandError(command, status, stderr_value, stdout_value)
git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
  cmdline: git log --merges origin/174-branchname..174-branchname
  stderr: 'fatal: ambiguous argument 'origin/174-branchname..174-branchname': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]''

ttwalto $ git publish 174-branchname
Publishing 174-branchname.
Branch '174-branchname' set up to track remote branch '174-branchname' from 'origin'.

ttwalto $ git sync
Pulling commits from the server.
Current branch 174-branchname is up to date.
Pushing commits to the server.

It would be helpful for sync to give a friendlier and more informative error, like:

Error: Branch 174-branchname is not published. Use a branch that is already published.

Start with the sync entry point in legit/cli.py, then follow smart_pull and smart_merge in legit/scm.py, as shown in the traceback. Reproduce the unpublished-branch case with git sync and make the failure report that the branch is not published instead of exposing a GitCommandError.】【。