bt2: prepend underscore to exceptions not meant to be raised by user

bt2: prepend underscore to exceptions not meant to be raised by user

The exceptions Error, MemoryError and LoadingError are only meant to be
raised by the Python bindings, in response to corresponding status codes
from the Babeltrace API.  As per our convention, names of classes not
meant to be instantiated directly by the user are prepended with an
underscore, so change these accordingly.

Note that they are still accessible to the user if they need to catch
an exception:

    try:
        ...
    except bt2._Error:
        ...

Change-Id: If094d817dac3c507b6bf3e1e794373f1c7fc33e4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1752
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>