snapcraft could be more helpful in figuring out prime/organize errs vs executable bits

the error message:

```
Failed to generate snap metadata: The command-chain item 'script.sh' defined in the app 'my-app' does not exist or is not executable.
Ensure that 'script.sh' is relative to the prime directory.
```

is not helpful when you are first creating a part that uses organize/prime/stage, etc. as it becomes unclear if the issue is your script is not executable or if the script was put in the wrong place or if the command-chain specification is wrong, making for needlessly confusing debugging, when snapcraft could be more specific saying that either the command-chain location and the file location do not match, and having a separate message about the script not being executable so that if your issue is just it not being executable you know that right away and don't have to spend time confirming that everything is put into the right place.