fix: update exception handling for AtLookup.findSecondary()

[srieteja]

Closes atsign-foundation/noports#483

- What I did

• update exception handling for AtLookup.findSecondary()

- How I did it

• split some very long exception messages into a log message and the rest as an exception message for more clarity
• add special exception handling for RootServerConnectivityException in CacheableSecondaryAddressFinder
• some more minor modifications to exception messages that are part of the finding secondary address process
• some minor formatting of exception messages and code comments in at_auth_impl

- How to verify it

• The updated exception message when the internet is disconnected should be Unable to authenticate | Cause: Unable to establish connection with root server. Please check your internet connection and try again

- Description for the changelog

fix: update exception handling for AtLookup.findSecondary()