Allow NewQueryEngine to return an error

Some errors only bubble up from pretty deep. A "SELECT COUNT(*)" was
ending up as a panic, when instead it should have been returned as a
query error to the user.

This adds plumbing for returning an error from NewQueryEngine.
uncomment some of the engine tests that tested invalid queries
Merge branch 'plumb-nqe-fail' of github.com:zorkian/influxdb

Conflicts:
	src/cluster/shard.go
Propagate the error back and assert on the body of the response
update the changelog