When client secrets are not base64 encoded, print an error message
that's not a generic base64 decode error:

   client secrets must be base64 decodable. See issue #337.
   Please consider replaceing "secret" with "c2VjcmV0"

When a user file is missing a mandatory field print an error message.

	Unable to build Server: user elroy-foo is missing email field

For #400