Clean up couch_replicator_httpc_pool pid on error

db_open cleans up the couch_replicator_httpc_pool process
if an error occurs accessing the remote Db.

BugzID: 27202

The slightly odd semantics here are because Db is used to refer to the httpc pool used to access the Db over http, so db_close is really stopping the httpc pool and not closing a db.