Add error output if command is not found

[koppor]

E.g., if texindy is not found

[gerking]

I encountered the problem that the environment variable XINDY_SEARCHPATH was not set up, so the invocation of texindy inside make-proceedings failed silently. There should be a proper error message in that case.

[koppor]

The only tool is texindy. When using latexmk in the first place, all tools are installed properly.

Not sure whether I should update to bash-task-runner or to bask.sh to ensure real proper error handling. I would say: latexmk does a good job, doesn't it?