Fix: bt2: replace bt2.CreationError instances with bt2._MemoryError

Commit

    4acc866e8298 ("bt2: rename CreationError to MemoryError, handle it in and out of Python bindings")

removed bt2.CreationError in favor of bt2.MemoryError (now
bt2._MemoryError).  However, some usages of bt2.CreationError were
introduced afterwards, probably by patches that were in the pipeline.
Replace them with bt2._MemoryError.

Change-Id: Id3cb6542e507b1e41c165ca61df21069d72fb44f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1867
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>