dnsdist: Better handling of exceptions raised during YAML parsing

dnsdist: Better handling of exceptions raised during YAML parsing

This commit changes the way exceptions raised during the YAML configuration
parsing are handled. I previously overlooked the way `cxx` handles exceptions
raised from a C++ code called from Rust:
```
If an exception is thrown from an extern "C++" function that is not declared
by the CXX bridge to return Result, the program calls C++'s std::terminate.
The behavior is equivalent to the same exception being thrown through a
noexcept C++ function.
```

Calling `std::terminate` is obviously not what we want, so this commit
declares that all C++ functions callable by Rust can raise exceptions,
and adds the required code to properly process these exceptions on the
Rust side of things.