Improve error handling

Added individual error classes
Don't silently fail for commands without callback from now on
General polishing (e.g. better error messages)

Fix typos

test/commands/getset.spec.js

@@ -32,7 +32,7 @@ describe("The 'getset' method", function () {
 
                 it('reports an error', function (done) {
                     client.get(key, function (err, res) {
-                        assert(err.message.match(/The connection has already been closed/));
+                        assert(err.message.match(/The connection is already closed/));
                         done();
                     });
                 });