STDERR is not properly ignored/dealt with (e.g. permission denied) about ack.vim

[kassio]

@tolry Thank's for reporting, and sorry about the late. You can change it on you own configuration changing the g:ackprg variable. If you, like me, would like this for default, could you please open a pull request to add this feature?

Thank's a lot.

[kassio]

One more information, ack has a option for that:

-s  Suppress error messages about nonexistent or unreadable files.  This is taken from fgrep.

However this option seems not be working on 2.12, I opened an issue about it there.