Nicer error messages

Right now, Ion throws error messages like this: ion: pipeline execution error: command exec error: Permission denied (os error 13)

While they are correct and tell you what the problem is, they could be more polished like rustc's errors (I'm only talking about the interactive prompt in this case).

No permissions

You don't have permission to open [FILE]. The owner is [USER].
Permissions are: rw--x---

Executables that can't be run

If it doesn't have any exec permissions:

Can't run [FILE]. It's not an executable.
Try running chmod +x [FILE].

If it has one, but we can't run it:

[FILE] is an executable but you don't have permission to run it. 
The owner is [USER]. Permissions are: rwx-w---
Try running chmod +x [FILE].

Command not found

Command not found: pyothn. Did you mean "python"?

If the command ends with a forward slash, it's a path not a command. The error should be more like:

Directory not found