Replace existing Exceptions in ring builder with more specific exceptions.
Abstracted out some behavior in ring-builder that is likely to cause an
exception. Add try/except blocks to swift-ring-builder to catch specific
exceptions and provide the user with some information about how to deal
with the error.
This change begins to address blueprint friendly-error-messages
Change-Id: I8fc9cfa4899421fe04bba23ac52523778e902321
2f50a0798e