neil dep add exits with code 0 on both failure and success

[joakimen]

When neil dep add fails to add a dependency, it still exits with code 0.

# bash

$ cat deps.edn
{}

$ neil dep add "com.cognitect.aws/s3"; echo $?
0

$ cat deps.edn
{:deps {com.cognitect.aws/s3 {:mvn/version "848.2.1400.0"}}}

$ neil dep add "some-fake-dep"; echo $?
0

$ cat deps.edn
{:deps {com.cognitect.aws/s3 {:mvn/version "848.2.1400.0"}}}

Discussion: https://clojurians.slack.com/archives/C03KCV7TM6F/p1688836219372089

[borkdude]

ok, I think printing a warning and then exiting with 1 would be ok