Suppress error messages for command line window

[KSR-Yasuda]

If command line window is opened with tabman window, it raises error messages for update tabman infomations.

Added silent! to avoid it.

How to Repro

1. Set let g:tabman_specials = 1 in .vimrc
2. Run vim
3. Open tabman window (<leader>mf)
4. Open command line window (q: or : -> <C-f>)
5. It raises errors in s:ManUpdate()

If g:tabman_specials is not set, it doesn't occur. Probably because command line window is not checked.