Bug fixes and added redis sentinel supportability changes.

src/main/java/tomcat/request/session/exception/BackendException.java

@@ -3,7 +3,7 @@ package tomcat.request.session.exception;
 /** author: Ranjith Manickam @ 3 Dec' 2018 */
 public class BackendException extends RuntimeException {
 
-    private static final String ERROR_MESSAGE = "For some reason, we could process your request. Please contact system administrator for more details.";
+    private static final String ERROR_MESSAGE = "For some reason we couldn't process your request. Please contact system administrator for more details.";
 
     public BackendException() {
         super(ERROR_MESSAGE, null, false, false);

src/main/java/tomcat/request/session/redis/SessionHandlerValve.java

@@ -24,12 +24,12 @@ public class SessionHandlerValve extends ValveBase {
 
     /** {@inheritDoc} */
     @Override
-    public void invoke(Request request, Response response) throws IOException, ServletException {
+    public void invoke(Request request, Response response) throws BackendException {
         try {
             getNext().invoke(request, response);
         } catch (IOException | ServletException | RuntimeException ex) {
             LOGGER.error("Error processing request", ex);
-            new BackendException();
+            throw new BackendException();
         } finally {
             manager.afterRequest(request);
         }