tmf: Avoid throwing exception in TmfTrace.buildStatistics()

tmf: Avoid throwing exception in TmfTrace.buildStatistics()

Replace the exception with an IStatus return value, similar to what was
done with .buildStateSystem().

Also handle the actual values that are returned in the caller (the
traceOpened signal handler) so that any eventual errors are logged.

Change-Id: Ia3b84aa6b195dc897193971e780058411050b59f
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/15072
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>