Currently the getConnection() in Elastica/Client.php can give false
positive and also throws error if no connection.

What is needed is a true/false check to allow for things like notice
feedback on extensions page or giving a warning when switching to
'elasticsearch' on site options page when it is unreachable.

Change-Id: I43506f03b943d79eb019d39fe794037fc50bd05a
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>