Fix SessionCleaner dying off due to uncaught exception. (#2031)

Because the run() method does not catch any exceptions, throwing an unchecked exception in checkExpiry propagates up and causes the SessionCleaner thread to die off.  Once this happens, browser sessions will no longer be closed due to their timeout.

This issue is described here:
SeleniumHQ/selenium-google-code-issue-archive#7248

java/server/src/org/openqa/selenium/remote/server/SessionCleaner.java

@@ -25,6 +25,7 @@
 import org.openqa.selenium.remote.server.log.PerSessionLogHandler;
 import org.openqa.selenium.support.events.EventFiringWebDriver;
 
+import java.util.logging.Level;
 import java.util.logging.Logger;
 
 class SessionCleaner extends Thread {   // Thread safety reviewed
@@ -115,7 +116,8 @@ void checkExpiry() {
           try {
             deleteSession.call();
           } catch (Exception e) {
-            throw new RuntimeException(e);
+            log.log(Level.WARNING, "Could not delete session " + session.getSessionId(), e);
+            continue;
           }
         }