same error for app command not existent and not executable

[Leo Arias]

When running snapcraft on a project with an unexisting command defined in an app, or when the command is not executable, the same error message is returned.

This is a problem because then there is no nice way to explain how to fix the problem. Instead of showing the same error, there should be one error for unexisting and a different one for not executable. That way the instructions on how to fix it will be clear and precise.

To reproduce, run snapcraft on a project with something like:

apps:
  test:
    command: idontexist

$ snapcraft
The specified command 'idontexist' defined in the app 'test' does not exist or is not executable

[Sergio Schvezov (sergiusens)]

https://github.com/snapcore/snapcraft/pull/2539