SEC-1939: Update SwitchUserFilter logger to use debug

Previously the SwitchUserFilter was logging as an error and then
throwing an Exception immediately after. This is not correct, since
whomever is catching the Exception should choose to log an error or not.

Now the log statement is at a debug level.

web/src/main/java/org/springframework/security/web/authentication/switchuser/SwitchUserFilter.java

@@ -250,7 +250,7 @@ public class SwitchUserFilter extends GenericFilterBean implements ApplicationEv
         Authentication original = getSourceAuthentication(current);
 
         if (original == null) {
-            logger.error("Could not find original user Authentication object!");
+            logger.debug("Could not find original user Authentication object!");
             throw new AuthenticationCredentialsNotFoundException(messages.getMessage(
                     "SwitchUserFilter.noOriginalAuthentication",
                     "Could not find original Authentication object"));