Add default human-oriented description for LoadError

[zhPavel]

Currently, string representation of LoadError contains only fields without human-friendly description

[bomzheg]

Hello, I was confused when misreaded NoRequiredFieldsLoadError as NotRequiredFieldsLoadError and this mismatch gave the opposite result (something like "error by loading existing not required field" instead of "not found required field"). It was my bad, but it will be cool if error will be more clear.

I have two ideas to improve it:

• rename error into something like RequiredFieldsNotFoundLoadError
• add field to error, like text="This error maybe means that the field does not exist"

discussion about this was here: https://t.me/reagento_ru/77088