MB-12251: Better error messages during emit

The errors on emit now are more specific. Their error atom is `key_emit`
instead of just `error`.

Change-Id: Id45f1a0a837ae2ddda12a7f9fd0cedf6a7359a91
Reviewed-on: http://review.couchbase.org/42753
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>