Warn the user if FSL does not find an executable
Currently, if FSL does not find the gmex/gmclash/fullsimlight executables, the relative button simply do nothing, without any warning to the user.
As such, the user does not know if there's a problem with the installation, the graphics, the SSH connection (if any), or anything else.
The issue is that FSL uses a plain, for example, gmex
command to launch GMEX from the GUI:
https://gitlab.cern.ch/GeoModelDev/GeoModel/-/blob/master/FSL/src/fsl_mainwindow.cpp#L1176
While that is fine when installing gmex with the global build and the default choices, it is not the case if the executable is installed in a non-system path.
Therefore, we should add a check to see if the executables are found; if not, we should warn the user about that.