Fix 'Undefined variable: message' in OCP\Util::logException

If the exception caught was *not* a subclass of `\Exception` it would echo `Undefined variable: message` because it tried to append data to an undefined variable.

This patch prevents that and slightly changes the log message in case of the exception being a subclass.

Before: `Exception: MyException: message`
Now: `MyException: message`

@PVince81 @MorrisJobke @karlitschek