feat!: raise FetchError instead of silently falling back

[steveclarke]

Summary

• Providers now raise Votd::FetchError instead of silently returning a default John 3:16 verse when fetch/parse fails
• Added Votd.logger for standard Ruby logging of provider errors
• Added Votd.on_error callback for error tracker integration (Sentry, Honeybadger, etc.)
• Original exception preserved as FetchError#cause for debugging
• CLI handles FetchError with user-friendly error output

Breaking change

All provider errors now raise Votd::FetchError instead of falling back to defaults. Users must rescue errors explicitly:

begin
  votd = Votd::BibleGateway.new(:niv)
rescue Votd::FetchError => e
  # handle error, use your own fallback, etc.
end

Test plan

• All 109 specs pass
• bundle exec ruby bin/smoke_test against live APIs
• bin/votd verse happy path
• bin/votd verse with network disconnected (should show error)

Closes #12