(autolab download) Exit code of 0 is returned on error

[victorhuangwq]

We observed this in 2 situations:
        1. when trying to download a non-existing assignment.
        2. when trying to download an already downloaded assignment.
           It's great that the CLI refuses to download the assignment if a directory
           with that name already exists, but it returns the same exit code as a
           successful download (which throws out our script).
        (There may be other situations, but we haven't run into them.)

https://github.com/autolab/autolab-cli/blob/master/src/cmd/cmdimp.cpp