Properly disable sound on errors on MacVim

[deathbeam]

Current configuration was not disabling that annoying sounds on MacVim. According to help:

Use visual bell instead of beeping.  The terminal code to display the
    visual bell is given with 't_vb'.  When no beep or flash is wanted,
    use ":set vb t_vb=".
    Note: When the GUI starts, 't_vb' is reset to its default value.  You
    might want to set it again in your |gvimrc|.

[amix]

Thanks!