Allow retries_exhausted to be defined in a block

sidekiq_retries_exhausted { |*args| }
Deprecate Worker#retries_exhausted in favor of the
`sidekiq_retries_exhausted` callback
Add link to Error-Handling wiki page
Pass the entire message to sidekiq_retries_exhausted
Merge pull request #1030 from jmazzi/feature/retries_exhausted_syntax_update

Allow retries_exhausted to be defined in a block