MB-8879: shrink error msgs exposed to UI and logs

Today when errors happened XDCR will dump errors to both xdcr log
and ns_server logs. Some information we dump to these log files is
redundant, causing bloated logs.

Also, XDCR now overwhelms UI ("last 10 errors" on XDCR tab) by
exposing too much info. The Erlang state from crash process is
too big to show on UI and it is unnecessary to show the complete
Erlang state to users.

This commit will shrink xdcr logs and the error msg exposed to UI.

Change-Id: I8863f20193794f52644bb09d804827ff454e3ede
Reviewed-on: http://review.couchbase.org/28256
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>