mon: OSDMonitor: don't ignore apply_incremental()'s return on UfP [1]

apply_incremental() may return -EINVAL.  Don't ignore it.

[1] UfP = Update from Paxos

Fixes: #5343

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>