BelugaException: empty callstack

[Masadow]

The normal behavior of BelugaException should return file + line + exception_message inside toString() method.

However, since CallStack.callStack() always returns an empty array, we can't locate where the problem happened.

I first thought that the code in toString() method was misplaced and should be inside the constructor. However, the CallStack.callStack() method still returns an empty array.

Maybe some macro tricks are needed but I didn't look deeper actually.