Improve CHUTNEY_PATH comments/errors

This had confused me. I was trying to use the chutney that was in my PATH when running ./tools/test-network.sh, so I thought that CHUTNEY_PATH would point to the chutney executable. Then I saw that it was supposed to be a directory, so I tried to point CHUTNEY_PATH to the directory containing the chutney executable, but that didn't work either. Now I see that CHUTNEY_PATH has to point to the chutney source code directory.

So this MR hopefully make it more clear that CHUTNEY_PATH needs to point to the chutney source code directory, and that it doesn't behave like most "PATH" environment variables.