MB-100: Made error message on bucket deletion more clear.

At the least this message should have "the bucket" instead of
bucket and end with a period to be more consistent.

I've made the response message a bit more clear overall though.

Change-Id: I7745e9a1ee8d3d0642176fa6a78ba39381dd2644
Reviewed-on: http://review.couchbase.org/23799
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>