Improve the error message when a checksum can't be found for a given
ruby.

This could be becuase the user hasn't updated their list of rubies so I
add a line to show how to update your list of rubies.
Merge pull request #1493 from KeeperPat/documentation/clearer-error-log-when-checksum-is-not-found

Improve the error message when a checksum can't be found for a given ruby.