cpp-common/bt2: add C++ bindings around `bt_error *`

cpp-common/bt2: add C++ bindings around `bt_error *`

Add C++ bindings to access the error on the current thread.

 - Take the current thread error with takeCurrentThreadError(),
   returning a `UniqueConstError`.
 - Move the current thread error back with moveErrorToCurrentThread().
 - Access the error cause by iterating or subscripting
   `UniqueConstError`, which yields objects of type `ConstErrorCause` (a
   borrowed object type).
 - Access the common error cause information using methods of
   `ConstErrorCause`, including the error cause actor type.
 - Obtain a more specific error cause object with
   `ConstErrorCause::as*()`.

Change-Id: Ie18e22ef8901bc9c8d7c85432d515ca763b484c2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12006
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>