wal-e-list-backups action fails if there are no backups available

The wal-e-list-backups action fails if there are no backups available:

$ juju run-action --wait postgresql/0 wal-e-list-backups
unit-postgresql-0:
  id: f7c08d45-7d54-4647-835d-a1fb2c826485
  message: Unhandled exception
  results:
    traceback: |
      Traceback (most recent call last):
        File "/var/lib/juju/agents/unit-postgresql-0/charm/actions/wal-e-list-backups", line 191, in main
          wal_e_list_backups(params)
        File "/var/lib/juju/agents/unit-postgresql-0/charm/actions/wal-e-list-backups", line 133, in wal_e_list_backups
          details = wal_e.wal_e_list_backups(envdir)
        File "/var/lib/juju/agents/unit-postgresql-0/charm/reactive/postgresql/wal_e.py", line 196, in wal_e_list_backups
          raw = wal_e_run(['backup-list', '--detail'], envdir=envdir)
        File "/var/lib/juju/agents/unit-postgresql-0/charm/reactive/postgresql/wal_e.py", line 192, in wal_e_run
          timeout=timeout)
        File "/usr/lib/python3.6/subprocess.py", line 336, in check_output
          **kwargs).stdout
        File "/usr/lib/python3.6/subprocess.py", line 403, in run
          with Popen(*popenargs, **kwargs) as process:
        File "/usr/lib/python3.6/subprocess.py", line 709, in __init__
          restore_signals, start_new_session)
        File "/usr/lib/python3.6/subprocess.py", line 1344, in _execute_child
          raise child_exception_type(errno_num, err_msg, err_filename)
      FileNotFoundError: [Errno 2] No such file or directory: '/snap/bin/wal-e.envdir': '/snap/bin/wal-e.envdir'
  status: failed
  timing:
    completed: 2019-06-05 12:42:10 +0000 UTC
    enqueued: 2019-06-05 12:42:08 +0000 UTC
    started: 2019-06-05 12:42:09 +0000 UTC
  unit: postgresql/0

I think that rather than failing it should complete and report an empty list.