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
Merge pull request #48 from cloudant/27202-fix-leaking-couch_replicator_httpc_pool-pids

Clean up couch_replicator_httpc_pool pid on error