bt2: print actual and expected lengths in _StaticArrayField._set_value error message

bt2: print actual and expected lengths in _StaticArrayField._set_value error message

I think it can help the user figure out what's wrong if we give the
lengths of the field and the passed value in the error message.

Change-Id: I94fb7ca04724d5d7af701df88aaedc195b1736fd
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2088
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>