TMF: Display a help text for analyses from the helper

TMF: Display a help text for analyses from the helper

When an analysis cannot be executed, it used to show the same message as if the
trace was not opened. Now it displays a verbose message explaining why the
analysis cannot be executed.

Change-Id: I97f1a79ac5c76362ebb9764b95fc8a7af99f963b
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/26295
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Hudson CI