Improve etc/setup_lxplus.sh

This MR follows up with some feedback I got during the BTTB8 tutorial on Corryvreckan:

When installing corry on lxplus, one has to clone the repo and then do

source etc/setup_lxplus.sh

to get the correct dependencies. However, this will create a misleading output, namely:

-bash: cd: /afs/cern.ch/user/j/jekroege/software/corryvreckan/etc/../bin: No such file or directory

Of course, this is correct because the software has not yet been compiled but it confused the new users. Therefore, I propose to add an if/else to check if the exectuable exists and if yes, continue as before, if not add an output message about how to proceed.