Packages should provide a way to validate credentials

when binding a package, often there are credentials provided by the user. it would be nice to **validate these credentials at bind time** so that we can **fail fast**, rather than failing in an ugly fashion when, later, the user decides to invoke actions in her bound package.

The issue does not name files, tests, or an entry point. Start by tracing package binding and credential handling in OpenWhisk, then determine how credentials can be checked during binding and what failure behavior should confirm that validation is complete.

Written by the indexing model from the issue text.