Module error 

Excel-style error representation that is both ergonomic now and flexible enough to grow new, data-rich variants later.

• ExcelErrorKind : the canonical set of Excel error codes
• ErrorContext : lightweight, sheet-agnostic location info
• ExcelErrorExtra: per-kind “extension slot” (e.g. Spill)
• ExcelError : one struct that glues the three together

When a future error needs its own payload, just add another variant to ExcelErrorExtra; existing code does not break.

Structs

ErrorContext

Generic, lightweight metadata that any error may carry.

ExcelError

The single struct your API passes around.

Enums

ExcelErrorExtra

Kind-specific payloads (“extension slot”).

ExcelErrorKind

All recognised Excel error codes.