Simplify some error handling and make it more structured

[MaxVerevkin]

• Remove kind and block fields from Error.
• Removed block field is replaced by BlockError struct.
• Represent formatting errors (such as "placeholder not found") as an enum with thiserror macro.
• Merge ResultExt and OptionExt into ErrorContext.

[MaxVerevkin]

Notable things that where broken and still are:

• Invalid block-specific configuration does not bring down the entire bar. But errors in the "common" config options (such as merge_with_next = 1 do). This is inconsistent. It might be okay to fail completely in both cases.
• Invalid click-handler button (such as button = "not a valid button") does not result in any sort of error.

[bim9262]

I think these changes look good. Let's create new issues for the things that you mentioned that are still broken (assuming there aren't already issues open)