Fix box.ctl.promote error handling

[yngvar-antonsson]

This pull request focuses on improving error handling and logging in the cartridge/failover.lua file. The most important changes include ensuring that errors from box.ctl.promote and box.ctl.demote are properly logged and handled, and that the raft_failover.disable function handles errors correctly.

Improved error handling and logging:

• cartridge/failover.lua: Changed the synchro_promote and synchro_demote functions to use pcall for error handling and added logging for errors. [1] [2]
• cartridge/failover.lua: Updated the reconfigure_all function to handle errors from synchro_promote by raising an error if synchro_promote fails.
• cartridge/failover.lua: Modified the cfg function to handle errors from synchro_promote, synchro_demote, and raft_failover.disable by raising ApplyConfigError if any of these functions fail. [1] [2] [3]
• cartridge/failover/raft.lua: Updated the disable function to use pcall for error handling and added logging for errors.

Documentation:

• CHANGELOG.rst: Added a note indicating that errors from box.ctl.promote and box.ctl.demote are now logged.

I didn't forget about

• Changelog