Fix: load: incomplete error handling for load_session_from_file

Fix: load: incomplete error handling for load_session_from_file

Observed issue
==============
lttng-ivc test fails to fail.

test_save_load_blocking_timeout[lttng-tools-2.12-lttng-tools-2.11-False]

Here we load a xml created by lttng-tools-2.12 and try to load it using
lttng-tools 2.11. We expect this to fail on the load.

The command report an error on the stderr but the command return code
value is zero.

From lttng-ivc test runtime.log:
Command #0
  Return value: 0
  Command: lttng load --input-path=/home/joraj/lttng/lttng-ivc/.tox/py3/tmp/test_save_load_blocking_timeou0/save_load saved_trace
  STDOUT:
      Session saved_trace has been loaded successfully
  STDERR:
      XML Error: Element 'process_attr_trackers': This element is not expected.
      Error: Session configuration file validation failed

Cause
==============
The error coming from load_session_from_file is not handled correctly.

Solution
========

Rework error handling in load_session_from_path and
load_session_from_file.

LTTNG_ERR_LOAD_SESSION_NOENT is NOT an error when session_name is
specified in load_session_from_path. In this scenario, we are actively
looking for the configuration of the session.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I0fea474045b718664e70ffe169e4e4fa125791a8