GUACAMOLE-323: Provide login help text during CAS redirect.

extensions/guacamole-auth-cas/src/main/java/org/apache/guacamole/auth/cas/AuthenticationProviderService.java

@@ -29,6 +29,7 @@ import org.apache.guacamole.GuacamoleException;
 import org.apache.guacamole.form.Field;
 import org.apache.guacamole.net.auth.Credentials;
 import org.apache.guacamole.net.auth.credentials.CredentialsInfo;
+import org.apache.guacamole.net.auth.credentials.GuacamoleInsufficientCredentialsException;
 import org.apache.guacamole.net.auth.credentials.GuacamoleInvalidCredentialsException;
 import org.apache.guacamole.auth.cas.conf.ConfigurationService;
 import org.apache.guacamole.auth.cas.form.CASTicketField;
@@ -89,7 +90,8 @@ public class AuthenticationProviderService {
         }
 
         // Request CAS ticket
-        throw new GuacamoleInvalidCredentialsException("Invalid login.",
+        throw new GuacamoleInsufficientCredentialsException(
+            "Please wait, redirecting to CAS authentication.",
             new CredentialsInfo(Arrays.asList(new Field[] {
 
                 // CAS-specific ticket (will automatically redirect the user