box: change error types for box_wait_quorum

Change error from "ER_QUORUM_WAIT: fiber is cancelled" to
FiberIsCancelled for consistency with other places that check for fiber
cancellation.
Same of "ER_QUORUM_WAIT: timed out". Change it to TimedOut.

While I'm at it, make box_wait_quorum tolerate spurious wake-ups.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring