Revert "be silent when opening files"

This reverts commit 6334e99192.
Opening files silently causes vim to hang if the file is already open by
another vim instance (it tries to print out the swap file
message/options but it cant).

plugin/NERD_tree.vim

@@ -2123,7 +2123,7 @@ function! s:OpenFileNode(treenode)
     else
         try
             wincmd p
-            silent exec ("edit " . a:treenode.path.StrForEditCmd())
+            exec ("edit " . a:treenode.path.StrForEditCmd())
         catch /^Vim\%((\a\+)\)\=:E37/
             call s:PutCursorInTreeWin()
             call s:Echo("Cannot open file, it is already open and modified")