bt2: remove NonexistentClockSnapshot exception type

bt2: remove NonexistentClockSnapshot exception type

The NonexistentClockSnapshot exception type is used when trying to get a
clock snapshot from an object that doesn't have clock snapshots (because
of the way it's configured).  It does not have much value over using a
standard exception type.  Replace it with ValueError.

In the test add some validation about the error message, because it
would be possible for the test to be a false positive if some ValueError
is raised for another reason (a programming error on our part, for
example).

Change-Id: Iebf86bc53e578407bb60f2145246dbd110cafaac
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1753
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>