cargo semver-checks might exit 0 if it finds only `publish = false` crates in a workspace

[rcoh]

1. Create a cargo workspace where all crates are set to publish = false
2. cargo semver-checks check-release -v Without -v it seems to imply that all is well—it will silently skip all crates. Checking 0 crates is unlikely to be what the user expected, perhaps it should exist non-zero?

[obi1kenobi]

I didn't add a test case specifically for workspaces full of publish = false crates, so I'm not going to close this right now, but #477 does add an explicit error if the selections contain no crates with library targets, so it's possible this case would be handled too.