tmf.swtbot: Add a failure message to 'isTableCellFilled'

tmf.swtbot: Add a failure message to 'isTableCellFilled'

This patch adds a failure message for the isTableCellFilled SWTbot
condition to tell what text was in the cell, if any and what was
expected

Change-Id: I8a3a8290af9a5a37c08ecf48cece6d67b1c96f44
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/84609
Reviewed-by: Hudson CI
Reviewed-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>