run.sh: don't silently ignore a non-executable file argument

run.sh: don't silently ignore a non-executable file argument

If we use a file which exists but is not executable as an argument to
run.sh, run.sh will silently ignore it. This is wrong.

Instead, just check for existence of the file (or directory), and error
out with a "Permission denied" message later, when we try to run it.