when a dependency fails verification, state why

[jessitron]

Currently dependency verification is true/false. There are multiple verifications, with different reasons for failing.

here : https://github.com/atomist/rug-resolver/blob/f759f5af30d988d48cb5a200f39b8977e5a45e3c/src/main/java/com/atomist/rug/resolver/DependencyVerifier.java#L8

... please return an optional error message instead of boolean. Then in DependencyWhitelistDependencyVerifier, say something nice like "Dependency xyz is not in the whitelist of extensions for team a."