Revert "Casbah 2.5.0, SalatDAO to use WR.getError"

This reverts commit 897a8f9019f285fe3f5285dc48e048d3bfc1c111.
On Rose's recommendation, back out SalatDAO changes and instead use a safer WriteConcern, like
WC.ACKNOWLEDGED.  This suggestion is also in line w/Mongo's direction as they will make this
writeconcern behavior the default going forward.
Rollback to prior version.

Use WC.ACKNOWLEDGED as default.  Mongo will go there soon anyway, and
code works fine as-is if you do.