JCBC-161: Remove duplication for observe checks

This changeset removes duplication in terms of observe node
size checks. A less accurate one has been removed in favor
of the - already in place - more accurate one.

Since the exception names have changed, the tests have been
adapted to reflect the new names. assumeTrue() fixes have
been added to make sure they don't produce false positives
in test scenarios which equal or more to 4 nodes.

Change-Id: Ie8b3b129b2b8781d916b54bc06f31f20408e7dad
Reviewed-on: http://review.couchbase.org/26261
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>