More graceful handling of failure in file execution

[dj3500]

Two examples:

• under Windows, we select not an .exe file but some other non-executable file (which exists). We then get "running..." indefinitely. Not sure if this is also broken under Linux (we have some sort of checking whether the file is executable, not sure if it works there).
• if the prepending command (like "java") is mistyped, or is not in the system path, the same happens.

What should happen: a useful error message.