CBQE-1340 Fix bugs in plotter.py

We should not halt the script if there is something wrong with
snapshot report generation because it will prevent us to get those
raw 90th, max stats. We can generate report manually if needed.
Also, if user forget to kick off any stats collector, it should not
affect the script.
Also some format change due to event database stats format change.
Change-Id: Ied0b718a16d8829e264cde41798162e0fa8327b0
Reviewed-on: http://review.couchbase.org/27689
Reviewed-by: Andrei Baranouski <linwindos@gmail.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>