[bind10-dev] Exceptions - nested vs on top of header

[feng]

For inner(nested) class, in c++, its usage is quite limited. In most case
they are used as private class which is only used by its outer class.

The big problem of public inner class is that it won't support forward
declare, for exception maybe this problem isn't obvious.

But to keeping code consistent and easy to read is important for big project
like bind10, and google c++ coding style including the usage of
Inner class, I think we can also include it into our coding style. This is
one topic similar with boost usage should be discussed in the coming
Face to face meeting.