cobalt - error count stats does not get incremented

SGW Version : All cobalt builds 

Steps to reproduce :

1. Set up one server and 2 sync gateways with the sgw provided 
2. delete sync gateway dbs
3. Do push replication from sgw1.DB1 to SGW.DB2
4. As DB are not existed, it should have errors and error count should get incremented.

Expected :

error count stats should get incremented while checking on both sgws using _expvars API. 

Actual:

Error count gets incremented on sgw1, but not on sgw2 . 

Error is thrown and get http exception , bug error counts stats are not getting incremented, warn count stats getting incremented though

SGW test on github: https://github.com/couchbaselabs/mobile-testkit/blob/master/testsuites/syncgateway/functional/topology_specific_tests/multiple_sync_gateways/test_sg_replicate.py#L315

command to run : 

pytest -s -rsx --mode=cc --server-version=6.0.1-2037 --sync-gateway-version=2.6.0-117 testsuites/syncgateway/functional/topology_specific_tests/ -k test_sg_replicate_non_existent_db

SGW logs : Attached .  errorcountstats.zip

^{Link to Jenkins failure : http://uberjenkins.sc.couchbase.com:8080/job/cen7-sync-gateway-functional-tests-topology-specific-cc/1805/testReport/testsuites.syncgateway.functional.topology_specific_tests.multiple_sync_gateways/test_sg_replicate/test_sg_replicate_non_existent_db/}