Log exceptions inside spawned functions

eventlet pool will discards all exception raised inside spawn(_n), it is
hard to discover problems inside spawned green-threads, it is better to
add a function wrapper to log the exceptions raised inside spawned
function.

Change-Id: I753df36c4614759f49134667a55f2a91f0c08317
Closes-bug: #1340708
Merge "Log exceptions inside spawned functions"