How to use the beets.ui.UserError function in beets

def test_art_file_missing(self):
        self.add_album_fixture()
        logging.getLogger('beets.embedart').setLevel(logging.DEBUG)
        with self.assertRaises(ui.UserError):
            self.run_command('embedart', '-y', '-f', '/doesnotexist')

"""Process Item `item`.
        """
        print_(format(item, fmt))
        if copy:
            item.move(basedir=copy, operation=MoveOperation.COPY)
            item.store()
        if move:
            item.move(basedir=move)
            item.store()
        if delete:
            item.remove(delete=True)
        if tag:
            try:
                k, v = tag.split('=')
            except Exception:
                raise UserError(
                    u"{}: can't parse k=v tag: {}".format(PLUGIN, tag)
                )
            setattr(item, k, v)
            item.store()

def build_alpha_spans(alpha_spans_str, alpha_regexs):
    """Extract alphanumerics from string and return sorted list of chars
    [from...to]
    """
    spans = []

    for elem in alpha_spans_str:
        if elem in alpha_regexs:
            spans.append(re.compile(alpha_regexs[elem]))
        else:
            bucket = sorted([x for x in elem.lower() if x.isalnum()])
            if bucket:
                begin_index = ASCII_DIGITS.index(bucket[0])
                end_index = ASCII_DIGITS.index(bucket[-1])
            else:
                raise ui.UserError(u"invalid range defined for alpha bucket "
                                   u"'%s': no alphanumeric character found" %
                                   elem)
            spans.append(
                re.compile(
                    "^[" + ASCII_DIGITS[begin_index:end_index + 1] +
                    ASCII_DIGITS[begin_index:end_index + 1].upper() + "]"
                )
            )
    return spans

def mb_call(func, *args, **kwargs):
    """Call a MusicBrainz API function and catch exceptions.
    """
    try:
        return func(*args, **kwargs)
    except musicbrainzngs.AuthenticationError:
        raise ui.UserError(u'authentication with MusicBrainz failed')
    except (musicbrainzngs.ResponseError, musicbrainzngs.NetworkError) as exc:
        raise ui.UserError(u'MusicBrainz API error: {0}'.format(exc))
    except musicbrainzngs.UsageError:
        raise ui.UserError(u'MusicBrainz credentials missing')

def mb_call(func, *args, **kwargs):
    """Call a MusicBrainz API function and catch exceptions.
    """
    try:
        return func(*args, **kwargs)
    except musicbrainzngs.AuthenticationError:
        raise ui.UserError(u'authentication with MusicBrainz failed')
    except (musicbrainzngs.ResponseError, musicbrainzngs.NetworkError) as exc:
        raise ui.UserError(u'MusicBrainz API error: {0}'.format(exc))
    except musicbrainzngs.UsageError:
        raise ui.UserError(u'MusicBrainz credentials missing')

def mb_call(func, *args, **kwargs):
    """Call a MusicBrainz API function and catch exceptions.
    """
    try:
        return func(*args, **kwargs)
    except musicbrainzngs.AuthenticationError:
        raise ui.UserError(u'authentication with MusicBrainz failed')
    except (musicbrainzngs.ResponseError, musicbrainzngs.NetworkError) as exc:
        raise ui.UserError(u'MusicBrainz API error: {0}'.format(exc))
    except musicbrainzngs.UsageError:
        raise ui.UserError(u'MusicBrainz credentials missing')

def mb_call(func, *args, **kwargs):
    """Call a MusicBrainz API function and catch exceptions.
    """
    try:
        return func(*args, **kwargs)
    except musicbrainzngs.AuthenticationError:
        raise ui.UserError(u'authentication with MusicBrainz failed')
    except (musicbrainzngs.ResponseError, musicbrainzngs.NetworkError) as exc:
        raise ui.UserError(u'MusicBrainz API error: {0}'.format(exc))
    except musicbrainzngs.UsageError:
        raise ui.UserError(u'MusicBrainz credentials missing')

def func(lib, config, opts, args):
            if not args:
                raise beets.ui.UserError('no device name specified')
            name = args.pop(0)
            
            items = lib.items(query=beets.ui.make_query(args))
            pod = PodLibrary.by_name(name)
            for item in items:
                pod.add(item)
            pod.save()
        cmd.func = func

a UserError if no items match. also_items controls whether, when
    fetching albums, the associated items should be fetched also.
    """
    if album:
        albums = list(lib.albums(query))
        items = []
        if also_items:
            for al in albums:
                items += al.items()

    else:
        albums = []
        items = list(lib.items(query))

    if album and not albums:
        raise ui.UserError('No matching albums found.')
    elif not album and not items:
        raise ui.UserError('No matching items found.')

    return items, albums