Unactionable error during builder validation

[zmackie]

In debugging #430 I attempted to create a config with duplicate CNBs removed. However this caused issues with versions, I think.
I got this rather cryptic error:

ERROR: validating buildpacks: buildpack org.cloudfoundry.node-engine@0.0.124 not found on the builder.

It would be nice to have something actionable out of that error. It reads like something for a developer to debug, rather giving the user something to fix.

[jromero]

Related to #306.

This is UX concern. I am all in favor for it and would like to discuss what the best strategy would be.

We could look at them on a case-by-case basis or, preferably, create a strategy for providing assistive text.

Initial ideas:

1. Create error types that could be asserted against at the cmd package and provide additional information there.
2. Create error types that have assistive text built into them that is displayed as part of error.Error().