mon/PaxosService: handle non-zero return values

If 7aec13f749035b9bef5e398c1ac3d56ceec8eb81 we started passing non-zero
return values to these completions; now we have to deal with them
accordingly.

RetryMessage behaves just like the Monitor variant.

Propose and Committed update state but otherwise ignore non-zero
return values.

Signed-off-by: Sage Weil <sage@inktank.com>