Make CouchbaseError.__str__ a bit nicer

Use a list and join it rather than appending strings

Change-Id: I8d22417aabba9b9b007066ebe2000b1866fa6ad4
Reviewed-on: http://review.couchbase.org/26686
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>