python: move exception types out of __init__.py

python: move exception types out of __init__.py

Move exception types defined in __init__.py.

 - Move _MemoryError to error.py
 - Move _IncompleteUserClass to component.py
 - Move the others to utils.py

I initially moved them to a new exceptions.py file, but I think that
it's just fine to move them in the existing files.  But it's a
possibility too, if y'all prefer that.

The goal of this commit is to make it easier to use these types
internally, using the form:

    from bt2 import utils as bt2_utils

    ... use bt2_utils.TryAgain ...

... in order to get rid of the internal `import bt2` imports.

Note that it would be possible to keep the exception types in
__init__.py and do:

    from bt2 import TryAgain

... and then use `TryAgain` in the code, but I think we prefer using a
module qualifier for whatever is imported.

Change-Id: I7d8dbdc28a3a2e3e38093903d622487a45fe7637
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10387
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>