message: serialization: Give explicit error message on context type mismatch.
diff: @@ -117,8 +117,16 @@ namespace elle
  auto it = this->_value.find(ti);
  if (it == this->_value.end())
  elle::err("missing serialization context for %s", ti.name());
  + try
  + {
  return boost::any_cast<T>(it->second);
  }
  + catch (boost::bad_any_cast const&)
  + {
  + ELLE_ABORT("unexpected context type {} instead of {}",
  + elle::type_info(it->second.type()), ti);
  + }
  + }
  /// Get @a value from the context.
  ///
  