Meaning of couchbase.PoolOverflow is now more intuitive

No effect on Sync Gateway; just making a corresponding change to one in
go-couchbase.
Return descriptive error on attempt to query views of non-healthy bucket

Just incorporating a new go-couchbase revision:
"If the CouchAPIBase property is missing, it's probably because the
bucket isn't in a healthy state, so report the state in the error."
For #147.