Create commands to existing filters should not be marked as failed.

Hello @andrew3886, @nicks, @eford1, @dpup, 

Please review the following commits I made in branch 'jamie-safe-create'.

A create to an existing filter would previously count as an error. This
could cause issues for safe commands being issued to the same filter
from multiple clients, due to a race condition.

771bda9bb74f7f0ec0b5260f18349b63cde63cba (2013-08-05 16:00:01 -0700)
Create commands to existing filters should not be marked as failed.


R=@andrew3886
R=@nicks
R=@eford1
R=@dpup