Fixes #429 - Add better error messages when credentials are not found.

* Create hackery to inspect registered credentials in the IvyCredentialStore.
* Create a new authenticator which inserts itself *after* the ivy authenticator.
  - Will issue an error message detailing host/realm required if credentials are
    not found.
  - Also lists out configured Realms with a 'is misspelled' message.
  - Ignores proxy-related authentication errors, for now.
Merge pull request #1147 from sbt/wip/issue-429

Fixes #429 - Add better error messages when credentials are not found.