ExceptionFormatter - Removed `FormatStack` in favor of exception's `.ToString()`

The `ToString` function of an exception already handles what you are trying to achieve with `FormatStack`, including inner exceptions. ref http://msdn.microsoft.com/en-us/library/system.exception.tostring.aspx

Sometimes custom exceptions may override `ToString()` to include important information. In this case, that information is lost when translated to Loggr.

I also fixed a unnecessary `null` check on `traceObject`.