tmf.core: Throw exception when no state system assigned

tmf.core: Throw exception when no state system assigned

Instead of just logging an error that will happen for each event and
willl be very verbose, it should throw an exception.

Change-Id: Iaedcf461ffa5e8d9ac8abd4f11e98e9f70f6259e
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/84035
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>