Handle malformed mountpoint directory input

[mharvilla]

valdi volume mount mounting-test ~?mnt
Traceback (most recent call last):
  File "/usr/lib/python3.8/pathlib.py", line 384, in gethomedir
    return pwd.getpwnam(username).pw_dir
KeyError: "getpwnam(): name not found: '?mnt'"

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/user/.venv/bin/valdi", line 8, in <module>
    sys.exit(main())
  File "/home/user/.venv/lib/python3.8/site-packages/valdi/entrypoint.py", line 34, in main
    ServiceRouter.route_to_service(args)
  File "/home/user/.venv/lib/python3.8/site-packages/valdi/router/service_router.py", line 20, in route_to_service
    volume_manager.mount_volume(args.volume_name, args.mountpoint)
  File "/home/user/.venv/lib/python3.8/site-packages/valdi/cli/volume_manager.py", line 38, in mount_volume
    mount_point = Path(mountpoint).expanduser()
  File "/usr/lib/python3.8/pathlib.py", line 1556, in expanduser
    homedir = self._flavour.gethomedir(self._parts[0][1:])
  File "/usr/lib/python3.8/pathlib.py", line 386, in gethomedir
    raise RuntimeError("Can't determine home directory "
RuntimeError: Can't determine home directory for '?mnt'