Fix misleading 'charmap' codec error on misspelled .cecli.conf.yml fields

[szmania]

The error 'charmap' codec can't decode byte 0x81 in position 3844: character maps to " occurs when a field is misspelled in the .cecli.conf.yml file.

For example, if agent-config is misspelled as gent-config, this misleading error appears.

We need to add a more intuitive error message and implement validation for the .cecli.conf.yml file to catch these misspellings early and provide a helpful error message to the user.