Make exception base class visible in basic_json

[ghost]

Let's make the base class of exceptions visible inbasic_json, along parse_error and others. The reason why I think it is useful is that often one wants to catch all exceptions related to a library in order to guarantee that they cannot escape module boundary. It also makes the code future-proof if new exception types are added. Of course, this can be done right now by simply catching nlohmann::detail::exception.

@nlohmann on a personal note, I'm sorry this comes after you've merged user-defined exceptions feature branch. I was traveling for the last couple of months and I did not pay attention. Sorry!

[nlohmann]

Fixed by merging #526.